For 1.5 hours, the boat passes between two piers against the course of the river, a distance of 18 km.

| January 15, 2021 | education

For 1.5 hours, the boat passes between two piers against the course of the river, a distance of 18 km. How long will it take to cover the return path if the river speed is 3 km / h.

t1 = 1.5 h.
S = 18 km.
VT = 3 km / h.
t2 -?
On the way back, the boat will follow the current, so the time of its movement t2 will be expressed by the formula: t2 = S / V2, where S is the distance between the berths, V2 is the speed of the boat behind the current.
V2 = V + Vt, where V is the speed of the boat relative to the water (own speed), Vt is the speed of the current.
t2 = S / (V + Vt).
The time of movement against the current t1 is expressed by the formula: t1 = S / V1, where V1 is the speed of the boat against the current.
V1 = V – Vt.
V1 = S / t1.
S / t1 = V – Vt.
V = S / t1 + Vt.
V = 18 km / 1.5 h + 3 km / h = 15 km / h.
t1 = 18 km / (15 km / h + 3 km / h) = 1 h.
Answer: the boat will sail in the opposite direction in time t1 = 1 hour.



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