Question

A boat traveled on a river 10 km upstream and then comes back to its starting point. Given that the trip takes 2 hours and the river has a current of 4 km an hour, write a quadratic equation that gives the boat's speed for the trip.

Responses
A 2x2 − 20x − 32 =0
B 2x2 − 8x − 32 =0
C 2x2 − 8x − 16 =0
D 2x2 − 16x − 32 =0

Answer 1

2x² - 20x - 32 = 0

Explanation:

Trip upstream:

distance = 10 km

time = t

current = 4 km/h

speed of boat in still water = x

speed upstream: x - 4

speed = distance/time

distance = speed × time

10 = (x - 4) × t

10 = tx - 4t

t(x - 4) = 10

t = 10/(x - 4)

Trip downstream:

Distance = 10 km

time = 2 - t

current = 4 km/h

speed downstream = x + 4

speed = distance/time

distance = speed × time

10 = (x + 4) × (2 - t)

10 = 2x - tx + 8 - 4t

10 = 2x - 10x/(x - 4) + 8 - 40/(x - 4)

10(x - 4) = 2x(x - 4) - 10x + 8(x - 4) - 40

10x - 40 = 2x² - 8x - 10x + 8x - 32 - 40

2x² - 20x - 32 = 0