Ed had 6 more marbles than Doug. Doug lost 7 of his marbles at the playground. If Ed had 22 marbles how many marbles does Doug have now?
Let's denote the number of Doug's marbles by d. Then the number of Ed's marbles is d+6. 

Doug lost 7 marbles, so now he has d-7 marbles. 

Ed had 22 marbles, so d+6=22. 

Therefore d=16 and Doug has 16 marbles now.