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Combinatorics 4.0/10

Lucky Tram Tickets

Tram tickets carry six-digit numbers from 000000 to 999999, leading zeros included, so there are one million tickets. A ticket is lucky if the sum of its first three digits equals the sum of its last three. How many tickets are lucky? Give an integer.

Related problems & prerequisites

Worth solving first

Source: The Soviet "lucky ticket" problem, a classic in wide circulation; no traceable original source. Statement written for AxiomIQ.