Question

a man rows upstream in still water at 3 km/h and back to the same place at 5 km/h he takes 48 minutes altogether. how far upstream did he go?​

Answer 1

Hey Dear,

◆ Answer -

Distance travelled upstream = 1.5 km

◆ Explanation -

Let s be the distance man go to. Also vm amd vr be velocity of man and that of riverstream respectively.

During upstream journey,

t1 = s / (vm-vr)

t1 = s / 3

During downstream journey,

t2 = s / (vm+vr)

t2 = s / 5

Total time taken is 48 min,

t = t1 + t2

0.8 = s/3 + s/5

0.8 = s × (3+5)/(5×3)

s = 1.5 km

Therefore, the man went 1.5 km upstream.

Thanks dear...

Answer 2

Solution:

Case 1

Speed = 3km/hr.

Let distance be x km

Speed = Distance / Time

Time = Distance / Speed

Time = x/3 hr.

Case 2

Speed = 5km/hr.

Let distance be x km

Speed = Distance / Time

Time = Distance / Speed

Time = x/5 hr.

Then,

(x/3) +(x/5) = 4/5

5x+3x/15 = 4/5

8x/15 = 4/5

5(8x) = 15×4

40x = 60

x = 60/40

x = 3/2 km or 1½ km