1/20 + 1/60 + 1/120 + … + 1/720
= 1/2 : 10 + 1/6 : 10 + 1/12 : 10 + ... + 1/72 : 10
= (1/2 + 1/6 + 1/12 + ... + 1/72) : 10
= (1 / 1 x 2 + 1 / 2 x 3 + 1 / 3 x 4 + ... + 1 / 8 x 9) : 10
= (1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/8 - 1/9) : 10
= (1/1 - 1/9) : 10
= 8/9 : 10 = 4/45