Originally Posted by Matyko

I admit I tried to make it the usual way of math but I failed with that. Then I just did the plain pen and paper solution starting from backwards: to have 15 left I need 31 watermelons, so when the half of it is gone, minus a half 15 will remain. 31 is double the 15 plus one. tested this for the next level and it worked: double 31 plus one: 63 is next level. counted back: worked. 63x2 + 1 = 127 for the last level, counted back, worked, tadaaam! I know it might sound stupid.. :P