1. 5
   15
2. 21
   2 cubed

A prime number is only divisible by itself and 1. Other prime numbers include 7, 11 and 17. They get rarer the farther you get from zero.

1. a decimal
   a rational number
2. a positive number
   an unknown number

"X" is an unknown number. In some equations, other letters may be pressed into service as other unknowns -- but "x" is always the first choice. It's where we get phrases like "X factor" or titles like "The X Files."

1. 4
   5
2. 6
   25

Six times six equals 36, minus 9 is 25. This is pretty simple algebra.

1. .111
   .333
2. .033
   3/10

Technically, the 3s should go on forever. This is because 1 can't be evenly divided by 3. Yet if you're dividing a cake into thirds, you can do it perfectly evenly. (Don't think about this too long; your brain will melt).

1. 1/8
   2/4
2. 4/5
   10/18

This would be easier to recognize as "8/10," which we could have easily used instead. But they're the exact same thing.

1. the denominator
   the numerator
2. the qualifier
   there is no name for this

Fractions consist of a numerator and a denominator. Fun fact: "denominator" has taken on a meaning in everyday English, in the expression "the common denominator," meaning something that unites seemingly different things or people. "Numerator" has no such casual use.

1. degrees
   points
2. sines
   cosines

"Degree" is a flexible word in the English language. We also use it to measure heat, to put a ranking on academic achievement (a master's degree), and to describe an undefined "certain amount" of something ("If you had even a degree of sympathy ...") And, of course, they're vital to geometry.

1. 45
   90
2. 180
   270

You'll find this question on many a basic geometry test. If one angle is 45, and the other is 90, then you know the third angle is 45 degrees.

1. circles
   waves
2. points
   exponents

Circles are perfectly round, but mathematicians think of them as implicitly divided up into any number of angles, from the center to any two points on the circumference. This is how we get compass readings, from 1 degree to 360 (or 0 to 359, if you prefer to count that way).

1. chord
   link
2. parallel
   x-y line

The circle's diameter counts as a chord. The radius does not, because it emanates out from the center point of the circle.

1. 8
   16
2. 64
   128

"Cubed" is the same way of saying "to the power of 3." 4 times 4 is 16, times 4 again is 64.

1. number rationality
   number irrationality
2. artistic notation
   scientific notation

Science often deals with very large numbers, and scientific notation makes them easier to manage. For example, in astronomy, it's easier to represent vast distances in light years using 10 and a superscript number than to write it out.

1. deponent
   exponent
2. proponent
   reponent

The word "exponent" has been hijacked by people who enjoy superlatives. "This stock is about to take off exponentially!" or "After I started reading Jordan Peterson, I saw exponential growth in my personal life!" Note to everyone: "exponential" does not just mean "a lot."

1. one millionth of a degree
   one millionth of a centimeter
2. one millionth of a line
   This is undefined.

Euclid said it best: "A point is that which has no part." He meant it was a theoretical concept. Likewise, a "line" in geometry has no specified measurement, either.

1. one
   two
2. three
   four

From a single point, you could move off in any direction. But just one other point, and suddenly you've got a line -- you can predict its path indefinitely. Just two points! Like a designer once said, "Simplicity is the ultimate style."

1. 90
   105
2. 125
   170

We covered this in an earlier question: The sum of the three interior angles of a triangle always add up to 180 degrees. Just wanted to see if you were paying attention!

1. acute
   exterior
2. obtuse
   none of these

Angles are classed as "acute" (less than 90 degrees) and "obtuse." Fun fact: Outside math, these words are generalized to mean "sharp" and "dull." At the emergency room, you might be asked if your pain is "acute." In the dating world, if you can't see that someone's just not that into you, you're being "obtuse."

1. right
   acute-right
2. obtuse-right
   either #2 or #3

A right angle is a 90-degree angle. It's a vital concept to engineering and architecture.

1. 100 degrees
   179 degress
2. 270 degrees
   360 degrees

Once an angle goes past 179 degrees to 180, well, it's a line. Beyond that, and it's an angle from the other direction (it might help to actually draw this on a piece of paper, to visualize it).

1. 1
   3.14
2. 7.28
   zero

The letter "pi" is a number used to determine the circumference or the area of a circle. The one for area is better known: A = pi times r(squared).

1. rabine
   radius
2. relativity
   all of these

The radius of a circle is half the diameter. Put another way, it's a line from the center of the circle to any point on the circumference.

1. dianthus
   diameter
2. demeter
   didjeridoo

The diameter is always twice the radius, and therefore can be used to find a circle's area. (Sidenote: If you chose "didjeridoo," wipe that smirk off your face; we know you knew better!)

1. the lowest common denominator
   a para-decimal
2. its singletons
   the fine print

The denominator is the key here, not the numerator. 5 is the lowest the denominator can go without being split into a fraction/decimal itself. On the other hand, there are plenty of higher denominators possible -- 20, 40, 100. However, reducing fractions to the lowest common denominator is a more common task in math than raising them to a higher common denominator.

1. 10.125
   10 1/8
2. 10 8/10
   10/80

8 does not divide into 81 evenly, so the answer could be written either as 10 1/8 or as 10.125. These are the same thing.

1. 1/3
   2/3
2. 66/100
   either #2 or #3

Like one-third, two-thirds would go on forever as a decimal: 0.666666 .... As a fraction, either 2/3 or 66/100 gets as close as you reasonably need to get.

1. a number to be divided
   a number that divides another number
2. the result of the division

In the simple equation 12 /4 = 3, 4 is the divisor. and 12 is the dividend. This is confusing because in everyday life, a "dividend" is the result of something being divided up: "The original investors shared the dividends."

1. 11
   12
2. 121
   the median is not shown, but it's 219

If you picked "219," you were confusing median with "mean," or average. But a median number is just one in the middle of a list. Here, that's 12.

1. two
   three
2. four
   five

There are five numbers on the list, so you'd divide the sum by five. Or, put another way: 103 divided by 5 equals 20.6. So 20.6 is the mean, or average, in this list.

1. a parallelogram
   a polygon
2. a rectangle
   a trapezoid

Don't try to slide this definition past a geometry teacher, though. The formal definition is a four-sided polygon with only two parallel sides.

1. 2
   3
2. 4
   5

3 times 3 is 9, times 3 again is 27. Subtract 5 from that, and you're left with 22.

1. 1
   2
2. 3
   7

This confuses new learners: on the left of a decimal point, the first digit represents single numbers, and the next one left of that is the tenths place. But to the right, the first place is the tenths place, and the next one hundredths. So here, 7 is in the hundredths place.

1. -4
   4
2. 1/4
   0

Integers are whole numbers, whether positive or negative, and zero. A fraction cannot be an integer.

1. circangle
   anglogram
2. radian
   sector

Frequently borrowed by sci-fi writers, "sector" is part of a circle. It's almost a triangle, but is not, because one side (the part defined by the circle's circumference), is curved.

1. itself plus one
   its distance from zero
2. itself to the power of ten
   itself to the power of one hundred

Whether the distance is positive or negative doesn't matter. That is, 4 and -4 have the same absolute value.

1. 2
   6
2. 7
   pi never ends

Pi is an irrational number; it goes on forever without repeating patterns. By general agreement, it is represented only as 3.14. Some math lovers pride themselves on being able to recite pi to dozens or hundreds of digits. We do not claim to understand this.