Answer:
2 km/h
Step-by-step explanation:
Let
Rate of river current = x km/h
Speed of Peter's boat = 4x km/h
He takes 15 km trip up (opposite direction)
His speed upstream = 4x - x
= 3x
His speed is 3x and distance is 15 km
Time = Distance / Speed
15/3x
= 5/x
And
Moving downstream (same direction)
Speed will be 4x + x = 5x
Distance = 15km,
speed = 5x
Time = Distance / Speed
= 15/5x
= 3/x
Total time take for trip = 4 hours
Total time take for trip = upstream + downstream
4 =5/x + 3/x
4 = 8/x
Cross product
4 * x = 8
x = 8/4
x = 2
Rate of river current = x km/h
= 2 km/h
