Hi, There is an equation for it. It's (1 + 40) x 40 : 2 = 820 Please note that the sum of the first (1) and the last (40) is 41. and the some pf the 2nd (2) and the 2nd last (39) is also 41. The sum of the 3rd (3) and the 3rd last (38) is also 41. So on. Now you have 20 pairs summing 41 each ...