Question

Smith sold 45 of his DVDs and purchased 10 more. If Smith has 15 DVDs now, how many DVDs did he have to begin with?

Answer 1

Let's set up a formula for this, where x is the amount of dvds he started with:

x-45+10=15
x-35=15
 +35  +35
x=50

He had 50 DVDs to begin with.

Check
50-45+10
=15

Answer 2

"x" represents the DVDs Smith began with:
(x-45)+10=15
Sold (-) 45, bought (+) 10, has (=) 15
Solve for x:
(x-45)+10-10=15-10
x-45=5
x-45+45=5+45
x=50