Example

Finding the Speed of a Car from 190-Foot Skid Marks

Problem: After a car accident, the skid marks for one car measured 190 feet. Using the formula s=24ds = \sqrt{24d}, find the speed of the car before the brakes were applied, rounding to the nearest tenth.

  1. Read the problem.
  2. Identify: The speed of the car.
  3. Name: Let ss = the speed in miles per hour.
  4. Translate: Write the skid marks formula and substitute d=190d = 190:

s=24190s = \sqrt{24 \cdot 190}

  1. Solve: Multiply inside the radical: 24×190=4,56024 \times 190 = 4{,}560. Evaluate the square root:

s=4,56067.5277s = \sqrt{4{,}560} \approx 67.5277\ldots

Round to one decimal place: s67.5s \approx 67.5.

  1. Check: Substitute back: 67.5?24(190)67.5 \stackrel{?}{\approx} \sqrt{24(190)}, so 67.5?4,56067.527767.5 \stackrel{?}{\approx} \sqrt{4{,}560} \approx 67.5277\ldots The approximation is consistent. Is 67.5 mph a reasonable speed for a car? Yes. ✓
  2. Answer: The speed of the car was approximately 67.5 miles per hour.

Because 4,5604{,}560 is not a perfect square, a calculator is needed to approximate the square root. The answer is then rounded to the specified decimal place. This example demonstrates that real-world applications of square root formulas often produce irrational results requiring approximation.

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Updated 2026-04-21

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