Track / Stage 4 / 144 of 340 #404 of 1964

Problem 404

AMC 12 late, AIME early

Combinatorics Difficulty 4.8 Find the answer

Ten people exchanged photographs. How many photographs were needed?

Your answer

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Official solution

21. The choice of one person can be made in ten ways, after which this person can give a photograph to nine remaining people. In total, $10 \cdot 9=90$ photographs are required.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.