Ed had 2 more marbles than Doug. Doug lost 24 of his marbles at the playground. If Ed had 29 marbles How many marbles does Doug have now?
Ed had 2 more marbles than Doug. 

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

Doug lost 24 of his marbles at the playground. 

That means that if we start with Doug's original number of marbles, and subtract 24, we'll get Doug's current number of marbles. 

Ed had 29 marbles. 

That means that if we start with Doug's current number of marbles, and add 2 (to get Ed's original number of marbles), and then add 29 (to get Ed's current number of marbles), we'll get Doug's current number of marbles. 

So: 

Doug's current number of marbles = Doug's original number of marbles + 2 + 29

Doug's original number of marbles = Doug's current number of marbles - 2 - 29

Doug's current number of marbles = (Doug's current number of marbles - 2 - 29) + 2 + 29

Doug's current number of marbles = Doug's current number of marbles

That means that Doug's current