A student's riddle hides the size of his own school inside its own answer
A traveller meets a group of students and asks how many attend their school. One replies: 'Count us twice, multiply that by three, divide the result into four equal parts, then add me to one of those parts — and you'll get 100.' How many students are there?
Reveal the answer
66 students. Working backwards from Alcuin's own 9th-century solution: (100 − 1) × 4 = 396, 396 ÷ 3 = 132, and 132 ÷ 2 = 66. Alcuin states the answer directly, 'twice 33 is 66,' and checks it rather than showing the algebra.
— Alcuin of York, Propositiones ad Acuendos Juvenes — c. 800 CE, Problem 48