Derek spends 4 out of every 7 dollars he earns on games. He used the rest of the money to buy puzzles. Last month, Derek spent 28 dollars on games. How many fewer dollars did he spend on puzzles?

Let the total amount of money he started with==M

4/7M == 28, solve for M

4M ==28 x 7

4M == 196

M == 196 / 4 ==$49 - total amount he started with

4/7 * 49 ==$28 - he spent on games

3/7 * 49 ==$21 - he spent on puzzles

28 - 21 ==$7 fewer dollars spent on puzzles.