This puzzle resource is a very difficult challenge and brings up to date this earlier resource (not tablet compatible) so that the same challenge can be worked with on all devices. It is simple enough to understand. Swap tiles so that each number in the triangle is the absolute difference between the two numbers below. The absolute difference is effectively the same as subtracting the smaller number from the larger number, whatever order they appear in.
The bottom numbers will always appear as correct as there are no numbers below to produce them. Note that the resource tells you when the absolute difference is correct but NOT that the tile is in the correct place to solve the puzzle.
Go to puzzle teaching resource.