Work Step by Step
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). The section header tells us to solve the equation using “addition or subtraction,” so we know that the only things we should do to solve this problem is addition or subtraction. 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. When we plug 11 in for f, we get that 20=20, so this answer is correct.