Berkeley Math Circle Monthly Contest 4 #0hk8

Berkeley Math Circle Monthly Contest 4Hard

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Understand the scale

Scale 0.5: Easiest math competition problems, often solvable even for early middle school students without experience. 1: Problems strictly for beginners, often at a moderate middle school level. 1.5: Problems for stronger beginner students, on the level of the middling problems in most middle school contests. 2: For motivated beginners, harder questions from the previous categories. 2.5: More advanced beginner problems, hardest questions from previous categories. 3: Early intermediate problems that require more creative thinking. 3.5: Tougher early intermediate problems that consistently stretch into higher-level creative thinking or conceptual knowledge. 4: Intermediate-level problems. 4.5: Upper intermediate problems approaching the upper bound for non-invitational sprint math competitions. 5: More difficult AIME problems or simple proof-based Olympiad-style problems. 6: High-level AIME-styled questions or introductory Olympiad-level questions. 7: Tougher Olympiad-level questions, may require more technical knowledge. 8: High-level Olympiad-level questions. 9: Expert Olympiad-level questions. 9.5: The hardest problems appearing on Olympiads which the strongest students could reasonably solve. 10: Historically notorious problems, generally unsuitable for even very hard competitions because they are exceedingly tedious, long, and difficult.

Counting two ways

Problem

Problem:

On a given street, there are $n$ houses numbered from $1$ to $n$ . Let $a_{i}$ ( $1 \leq i \leq n$ ) be the number of people living in the house numbered $i$ , and let $b_{i}$ ( $i \geq 1$ ) be the number of houses on the street in which at least $i$ people live. Prove that

a_{1}+a_{2}+\cdots+a_{n}=b_{1}+b_{2}+b_{3}+\cdots

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