156850

First of all, you need to find the total from 1-20 in order to find the sum in each pile. 1+2+3+4 (etc. up to 20)= 210. Then you divide that by six (because of the six piles)= 35.
I started using bigger numbers in each pile.
So:

20+14+1
19+13+3
18+12+5
17+11+7
For the last two piles, I swapped the second numbers around to make 35:
16+9+4+6
15+10+2+8

Each pile adds up to 35.