Example

Solving a Uniform Motion Application: Boat Speed with Equal Travel Times

Apply the distance, rate, and time problem-solving strategy to find an unknown boat speed when different distances are covered in equal time—one segment upstream and one downstream.

Problem: On a river that flows at 77 miles per hour, Danica can take a motorboat 55 miles upstream in the same amount of time she can take her motorboat 1212 miles downstream. How fast is Danica's boat in a lake?

  1. Read and draw: Sketch the river paths — 55 miles upstream and 1212 miles downstream. Create a rate–time–distance table.
  2. Identify: The boat's speed in still water (a lake).
  3. Name: Let rr = the boat's speed in still water. The upstream rate is r7r - 7; the downstream rate is r+7r + 7. Because t=Drt = \frac{D}{r}, the times are 5r7\frac{5}{r - 7} and 12r+7\frac{12}{r + 7}.
  4. Translate: The travel times are equal: 5r7=12r+7\frac{5}{r - 7} = \frac{12}{r + 7}
  5. Solve: Multiply both sides by the LCD, (r7)(r+7)(r - 7)(r + 7): 5(r+7)=12(r7)5(r + 7) = 12(r - 7) 5r+35=12r845r + 35 = 12r - 84 119=7r119 = 7r r=17r = 17
  6. Check: Upstream time: 5177=510=12\frac{5}{17 - 7} = \frac{5}{10} = \frac{1}{2} hour. Downstream time: 1217+7=1224=12\frac{12}{17 + 7} = \frac{12}{24} = \frac{1}{2} hour. The times are equal. \checkmark
  7. Answer: Danica's boat speed in a lake is 1717 mph.

0

1

Updated 2026-05-01

Contributors are:

Who are from:

Tags

OpenStax

Intermediate Algebra @ OpenStax

Ch.7 Rational Expressions and Functions - Intermediate Algebra @ OpenStax

Algebra

Related