Ed had 6 more marbles than Doug. Doug lost 10 of his marbles at the playground. If Ed had 4 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 10 of his marbles at the playground. 

That means that if we start with Doug's number of marbles, then subtract 10, we'll get Doug's current number of marbles. 

Ed had 4 marbles. 

That means that if we start with Ed's number of marbles, then subtract 4, we'll get Ed's current number of marbles. 

Now let's put it all together. If we start with Doug's number of marbles, then subtract 10, we'll get Doug's current number of marbles. And if we start with Doug's number of marbles, then add 6, we'll get Ed's number of marbles. So if we take Doug's current number of marbles and add 6, we'll get Ed's current number of marbles. And we know that Ed's current number of marbles is 4. So Doug's current number of marbles is 4 - 6, or -2.