Ruby isn't limited to simple expressions of assignment like my_num = 100; it can also do all the math you learned about in school.
There are six arithmetic operators we're going to focus on:
Addition (+)
Subtraction (-)
Multiplication (*)
Division (/)
Exponentiation (**)
Modulo (%)
The only ones that probably look weird to you are exponentiation and modulo. Exponentiation raises one number (the base) to the power of the other (the exponent). For example, 2**3 is 8, since 2**3 means "give me 2 * 2 * 2" (2 multiplied together 3 times). 3**2 is 9 (3 * 3), and so on.
Modulo returns the remainder of division. For example, 25 % 7 would be 4, since 7 goes into 25 three times with 4 left over.