It is called a sum or an equation
When 3 is added 5 times to a number the result is 15 if the number is 0, otherwise this is not true. However, the amount added to the number is always 15. Let's say the variable x is our number and y is the result, then x+3+3+3+3+3=y. You can add all the 3's together now. So you have x+15=y. Now if the starting number is 0, you have 15=y, so the result is 15.
To balance a equation, the whole number added or used in front of a compound is known as Coefficient.
The term for the number added in front of some compounds to balance a chemical equation is the coefficient.