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?
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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 7 marbles. 
That means that if we start with Doug's current number of marbles, and subtract 7, we'll get Doug's original number of marbles. 
Ed had 22 marbles. 
That means that if we start with Ed's current number of marbles, and subtract 22, we'll get Ed's original number of marbles. 

Now let's put it all together. If we start with Ed's original number of marbles, and subtract 22, then add 6, then subtract 7, we'll get Doug's original number of marbles: 

Doug's original number of marbles = Ed's original number of marbles - 22 + 6 - 7

We know that Ed had 6 more marbles than Doug originally, so we can say that: 

Ed's original number of marbles = Doug's original number of marbles + 6

Now we can plug this into our equation for Doug's original number of marbles: 