Chapter 2 - Solving Equations - 2-1 Solving One-Step Equations - Practice and Problem-Solving Exercises - Page 85: 14

f=11

Addition In order to solve this algebraic expression, we must isolate the variable. A variable is the letter in the problem (for instance, in the equation x+2=10, x is the variable). Because addition and subtraction are inverse operations (they cancel each other out), we know we must add when a number is being subtracted from the variable and subtract when a number is being added to the variable. In the equation f+9=20 we must get rid of the 9, which is on the same side as f, in order to solve. Thus, we subtract 9 on both sides of the equation to find that f=11. We plug 11 in for f in our original equation and confirm that our answer is, indeed, correct.