A motorboat covers the distance between two spots on the river banks `t_(1)=8g` and `t_(2)=12h` in down stream and upstream respectively. The time required for the boat to cover this distance in still water will be

Options

A. `6.9h`
B. `9.6h`
C. `69s`
D. `96s`

Explanation

(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`