1/30+1/42+1/56+1/72
=1/5*6+1/6*7+1/7*8+1/8*9
=(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)
=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9
=4/45
1/30+1/42+1/56+1/72
=1/5*7+1/6*7+1/7*8+1/8*9
=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9
=9/45-5/45
=4/45
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