1+2-3+4+5-6+7+8-9+...........+58+59-60

=( 1+2-3)+(4+5-6)+(7+8-9)+...+(58+59-60)=0+3+6+9+...+57

So a * * * has 20 items, the tolerance is 3, and the first item is 0.

And =(0+57)X20/2=570.