MathDB

2021 Spring LMT Division B Problem 13

Source:

January 22, 2022

number theory

Problem Statement

Call a

4

-digit number

\overline{a b c d}

unnoticeable if

a +c = b +d

and

\overline{a b c d} +\overline{c d a b}

is a multiple of

7

. Find the number of unnoticeable numbers.
Note:

a

b

c

, and

d

are nonzero distinct digits.
Proposed by Aditya Rao