To solve this problem you have to draw bar diagrams that represent the situation.
Gabriel
Daniel
1. They begin with the same amount of money.
2. After they spend money Daniel has 2 times the amount Gabriel now owns.
Daniel spends $10
Gabriel spends $15
This is the part we need to find. Gabriel spends more. As you can see the new amount is  15 - 10.
5
5
5
5
10
10
Daniel
Gabriel
The unit on the bar diagrams is 5. Now we can easily find the answer to the problem.
Your answer is= 20.
Test it now:
Daniel spends 10
20-10=10
Gabriel spends 15
20-15= 5
Daniel has now twice as much as Gabriel.
You got the right answer!
And with the help of a bar diagrams.
Bar Diagrams
10
15