Giải thích các bước giải:
`M=1/4+1/10+1/18+1/28+1/40+1/54+1/70`
`=> M = (1/4+1/28) + (1/10+1/70) + (1/18+1/54) + 1/40`
`=> M = (7/28+1/28) + (7/70 + 1/70) + (3/54 + 1/54) + 1/40`
`=> M = 8/28 + 8/70 + 4/54 + 1/40`
`=> M = 2/7 + 4/35 + 2/27 + 1/40`
`=> M = 2/5 + 2/27 + 1/40`
`=> M = (16/40 + 1/40) + 2/27`
`=> M = 17/40 + 2/27`
`=> M = 539/1080`