$#Dino$
a) `C1`
`(5/4+2/5):5/6`
`=33/20:5/6`
`=99/50`
`C2`
`(5/4+2/5):5/6`
`=5/4xx6/5+2/5xx6/5`
`=3/2+12/25`
`=99/50`
.................................
`b) C1`
`3/4xx2/5-3/4xx2/7`
`=3/10-3/14`
`=3/35`
`C2`
`3/4xx2/5-3/4xx2/7`
`=3/4xx(2/5-2/7)`
`=3/4xx4/35`
`=3/35`
..................................
`c) 2/3:(5/6xx9/10)`
`=3/2xx3/4`
`=9/8`
`C2`
`2/3:(5/6xx9/10)`
`=2/3:5/6xx9/10`
`=2/3xx6/5xx9/10`
`=9/8`