Teaser 2977: Enjoying retirement

From The Sunday Times, 13th October 2019 [link]

George and Martha have worked on separate departments of a company which has four-digit telephone extensions. George looked at his extension and it was ABCD. Martha’s (larger) also had A, B and C as her first three digits but not necessarily in that order. Her last digit was E. They added up their two four-digit numbers and found that the least significant digit was F. They then looked at the difference and that was a four-digit number of which the least significant digit was G. They then looked at the product and the least significant digit was H. They then looked at the average of the extensions; in it the first two digits were equal, the last two digits were also equal, and the least significant digit was J. I have thus mentioned nine digits, all positive and unequal.

What was Martha’s extension?

[teaser2977]