1.

A motor boat, whose speed is 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hrs 30 mins. The speed of the stream (in km/hr) is:-1. 5 km/hr.2. 10 km/hr.3. 15 km/hr.4. 20 km/hr.5. 25 km/hr.

Answer» Correct Answer - Option 1 : 5 km/hr.

Given:

Speed in still water = 15 km/hr

Total distance = 30 km

Total time = 4 hrs 30 mins = 412 hr = 9/2 hr.

Formula used:

Distance = Speed × Time 

Calculation:

Let the speed of the of the stream be X km/hr

Speed upstream = (15 – X) km/hr

Speed downstream = (15 + X) km/hr 

According to question:

[30/(15 + x) + 30/(15 X)] = 9/2

(450 30X + 450 + 30X) / (15 + X) × (15 X) = 9/2

900 = [(152 – X2) × 9/2]

(900 × 2/9) = 225 – X2

X2 = 225 200

X2 = 25

x = 5

Speed of the stream is 5 km/hr.

If the speed of boat or swimmer is X km/hr and the speed of the stream is y km/hr then,

Speed of boat upstream = (X Y) km/hr.

Speed of boat downstream = (X + Y) km/hr.



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