(b) Let the distances between spots `A` and `B` is `l`
`u=` speed of river stream
`v=` speed of motorboat in still water
During downstream
`t_(1)=l/(v+u)`…………(i)
similarly in upstream
`t_(2)=l/(v-u)`……….(ii)
In still water, `t=l/v`...............(iii)
After solving eqs (i),(ii) and (iii) we get `t=9.6h`