Sunday Times Teaser 3014 – Family Business

by Danny Roth

Published Sunday June 28 2020 (link)

George and Martha run a company with their five daughters. The telephone extensions all have four positive unequal digits and strangely only four digits appear in the seven extensions:
\[\begin{array}{|l|r||}
\hline Andrea & abcd \\
\hline Bertha & acdb \\
\hline Caroline & bacd \\
\hline Dorothy & dabc \\
\hline Elizabeth & dbca \\
\hline George & cabd \\
\hline Martha & cdab \\
\hline \end{array}\]

They noticed the following:

Andrea’s and Bertha’s add up to Dorothy’s.

Bertha’s and Elizabeth’s add up to George’s.

Caroline’s and Dorothy’s add up to Martha’s.

What is Andrea’s extension?