A boat travels 8 km upstream and 8 km back. The time for the round trip is 5 hrs. The speed of the stream is 2 ​km/hr. What is the speed of the boat in still​ water?

Answer 1

8/(x+2) + 8/(x-2) =5

solve for x = about 4.16 kmh = boat speed in still water

multiply by x^2-4 to eliminate fractions

8(x-2) + 8(x+2) = 5(x^2-4)

8x -16 + 8x +16 = 5x^2 -20

5x^2 -16x -20 = 0

use the quadratic formula

x = 16/10 (1/10)sqr(16^2 +400) = 1.6+.2sqr656 = about 4.16

x = 4.15 km per hour = speed of the boat in still water.