15/56=1/7+1/8
2/63=1/7-1/9
1/24=2/48=1/6-1/8
19/90=1/9+1/10
所以原式=(1/7+1/8)-(1/7-1/9)+(1/6-1/8)-(1/9+1/10)
=1/7+1/8-1/7+1/9+1/6-1/8-1/9-1/10
=1/6-1/10
=1/15