7乘15分之8-7乘35分之12加7乘63分之16-99分之20乘7
=7(8/15-12/35+16/63-20/99)
=7x{(1/3+1/5)-(1/5+1/7)+(1/7+1/9)-(1/9+1/11)}
=7x(1/3-1/11)
=7x8/33
=56/33