A = 1x2 + 2x3 + 3x4 + 4x5 + ...+ 59x60
A x 3 = 1x2x3 + 2x3x3 + 3x4x3 + 4x5x3 + ... + 59x60x3
A x 3 = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + 4x5x(6-3) + ... + 59x60x(61-58)
A x 3 = 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 + 4x5x6 - 3x4x5 + ... + 59x60x61 - 58x59x60.
A x 3 = 59x60x61
A = 59x60x61 : 3
A = 71980