Question

Meg rowed her boat upstream a distance of 32 mi and then towed back to the starting point. The total time of the trip was 18 hours. If the rate of the current was 7 mph, find the average speed of the boat relative to the water.

Answer 1

Answer: the average speed of the boat relative to the water is 9 mph

Step-by-step explanation:

Let x represent the speed of the boat.

The rate of the current was 7 mph,

Assuming the boat travelled against the current while going upstream, the total speed would be x - 7

Assuming the boat in the direction of the current while going downstream, the total speed would be x + 7

Time = speed × time

Since she travelled 32 miles upstream and 32 miles downstream, then,

Time taken to go upstream = 32/(x -7)

Time taken to go downstream = 32/(x + 7)

Since the total time of the trip was 18 hours, then

32/(x -7) + 32/(x + 7) = 18

Cross multiplying by (x - 7)(x + 7), it becomes

32(x + 7) + 32(x - 7) = 18[(x - 7)(x + 7)

32x + 224 + 32x - 224 = 18(x² + 7x - 7x - 49

32x + 32x = 18x² - 882

18x² - 64x - 882 = 0

Dividing through by 2, it becomes

9x² - 32x - 441 = 0

9x² + 49x - 81x - 441 = 0

x(9x + 49) - 9(9x + 49) = 0

x - 9 = 0 or 9x + 49 = 0

x = 9 or x = - 49/9

Since the speed of the boat cannot be negative, then x = 9