MathDB
Combi With Digits

Source: PRMO Leg 2 P20

August 25, 2019
combinatoricsPRMO

Problem Statement

How many 4−4-digit numbers abcd‾\overline{abcd} are there such that a<b<c<da<b<c<d and b−a<c−b<d−cb-a<c-b<d-c ?
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