To solve this problem first you have to make a diagram of the situation
1. Gabriel has twice as many as Peter. And Peter has 20 more than Daniel. Use bars to represent the situation.
Gabriel
200
Peter
Daniel
Daniel
20
?
2. Peter is Daniel + 20 and Gabriel is 2 times Peter. So that Daniel is the part that we need to find out. Let's place this unit (Daniel) on Peter's bar. Peter is now Daniel + 20. Now go to Gabriel. Peter fits twice in it.
Daniel
20
Daniel
20
Peter
Daniel
20
Daniel
Gabriel
200
Daniel
3. Unit Daniel repeats 4 times . To find how many cards Daniel has:
We subtract from 200 all the known values: 20+20+20=60
200-60= 140,  Daniel 4 times
140:4= 35,  Daniel
If you know how to use parenthesis this is the same as:
(200 - 60) : 4
 = 140 : 4
 =  Daniel has 35 cards

Remember 140 is Daniel 4 times.

Peter has
35 + 20 = 55 cards

Finally:

Gabriel has
55 * 2 = 110 cards
Bar Diagrams
35
20
35
20
Peter
35
20
35
Gabriel
200
Daniel
Peter is Daniel + 20
The bar that represents Gabriel is 2 times as long as Peter's
The problem solved: