A boy, a plank, and just enough bricks to balance himself
A boy wants to see-saw alone, so he ties bricks to the other end of the plank to balance his own weight. Tied to the plank's short end, it takes 16 bricks to balance him; tied to the long end, only 11 bricks do it. A brick happens to weigh exactly three-quarters of a brick plus three-quarters of a pound. What is the boy's weight?
Reveal the answer
About 39.8 lbs. First, a brick's own weight: b = ¾ b + ¾ lb gives b = 3 lbs, so 16 bricks weigh 48 lbs and 11 weigh 33 lbs. Dudeney's rule: multiply the two totals and take the square root — √(48×33) ≈ 39.79 lbs, the boy's weight. From Henry Dudeney's Amusements in Mathematics (1917), puzzle No. 122, "The See-Saw Puzzle."
— Henry Dudeney, Amusements in Mathematics — Puzzle No. 122, "The See-Saw Puzzle," 1917