15 = 3 * 5
51 = 3 * 17
1/15 = 1/(3 * 5)
1/51 = 1/(3 * 17)
原式 = [1/(3*5) - 1/(3*17)] * 17 * 5 = (17*5)/(3*5) - (17*5)/(3*17) = 17/3 - 5/3 = 12/3 = 4