Jack swam directly across the pool, diagonally from one corner to another. Which of these is the best estimate of how far he swam?

• A. 15 feet
• B. 25 feet
• C. 45 feet
• D. 35 feet

Answer: (B)

When you swim from one corner of a rectangle to the opposite corner, you’re measuring the diagonal, which is found using the right-triangle relationship: diagonal^2 = length^2 + width^2. If the pool is 15 feet by 20 feet, the diagonal is sqrt(15^2 + 20^2) = sqrt(225 + 400) = sqrt(625) = 25 feet. So the distance across is 25 feet, making that choice the best estimate. The numbers 35 and 45 would come from adding sides or other combinations that don’t match the diagonal of a 15 by 20 rectangle.