Ed had 6 more marbles than Doug. Doug lost 2 of his marbles at the playground. If Ed had 45 marbles How many marbles does Doug have now?
Ed had 6 more marbles than Doug. 

That means that if we start with Doug's number of marbles, then add 6, we'll get Ed's number of marbles. 

Doug lost 2 of his marbles at the playground. 

That means that if we start with Doug's number of marbles, then subtract 2, we'll get Doug's current number of marbles. 

Ed had 45 marbles. 

That means that if we start with Doug's number of marbles, then add 6, we'll get 45. 

So, if we start with Doug's number of marbles, subtract 2, and add 6, we'll get 45. That means Doug's current number of marbles is 39.