Question

1/20+1/30+1/42+1/56+1/72

Answer 1

1/20+1/30+1/42+1/56+1/72

=1/4x5+1/5x6+1/6x7+1/7x8+1/8x9

=1/4-1/5+1/5-1/6+...+1/9

=1/4-1/9

= 5 / 36

Answer 2

= 1/4x5 + 1/5x6 + 1/6x7 + 1/7x8 + 1/8x9
=1/4 - 1/5 + 1/5 - 1/6 +... + 1/8 - 1/9
= 1/4 - 1/9 = 5/36