MathDB
Easy combinatorics about finding minimum integer

Source: BdMO 2022 Secondary P4

April 12, 2022
combinatorics

Problem Statement

Pratyya and Payel have a number each, nn and mm respectively, where n>m.n>m. Everyday, Pratyya multiplies his number by 22 and then subtracts 22 from it, and Payel multiplies his number by 22 and then add 22 to it. In other words, on the first day their numbers will be (2n2)(2n-2) and (2m+2)(2m+2) respectively. Find minimum integer xx with proof such that if nmx,n-m\geq x, then Pratyya's number will be larger than Payel's number everyday.