The ratio of speed of a boat in downstream to upstream is 5 3 and total time taken by boat to cover ...........

Explanation:

Let speed of boat in still water and speed of stream be x km/hr and y km/hr respectively

3x + 3y = 5x – 5y
2x = 8y
x : y = 4 : 1
ATQ
Let speed of boat in still water = 4s km/hr
And speed of stream = s km/hr
30/3s = 5
s = 2 km/hr
Required time = 40/(5×2) = 4 hours
Alternate method

Upstream speed of boat= 30/5=6km/hr
3x=6=> x=2
Downstream speed= 5x= 5×2= 10km /hr
Time taken by boat in downstream= 40/10=4 hrs.