Answer:
A. 20.81 mph
Step-by-step explanation:
x = speed of the boat in still water.
one part of the trip is 10 miles downstream, where the speed of the boat and the speed of the current add together.
and the other part of the trip is the trip back the 10 miles upstream, where the speed of the current works against the speed of the boat :
10/(x + 15) + 10/(x - 15) = 2
10 + 10×(x+15)/(x-15) = 2×(x+15)
10×(x-15) + 10×(x+15) = 2×(x+15)×(x-15)
20x = 2×(x² - 225)
10x = x² - 225
0 = x² - 10x - 225
solution of a quadratic equation
x = (-b ± sqrt(b² - 4ac)) / (2a)
a = 1
b = -10
c = - 225
(10 ± sqrt(100 - 4×1×-225)) / 2 =
= (10 ± sqrt (100 + 900)) / 2 = (10 ± sqrt(1000)) / 2 =
= (10 ± 31.62) / 2
x1 = 41.62 / 2 = 20.81 mph
x2 is negative and did not make any sense.
