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I solved the 'build it up' problem and found some easy patterns of doing it.
One of the ways of doing it was that if you found was look at the incorrect ones you can change the bottom numbers around. For example, if you had 4,1,2 and 3 and they did not work out, you could change them around to 1,3,4 and 2 (I'm not saying that it is correct, it is only an example!). One other way is think of other ways of making 15 e.g 5+10 and work your way down from the top.