Question 24
The letters $A,$ $B,$ $C$ and $D$ all represent different digits. If $ \begin{array}{ccc} &A &B \\ + &C &A \\ \hline &D &A \end{array} $ and $ \begin{array}{ccc} &A &B \\ - &C &A \\ \hline &&A \end{array}, $ what digit does $D$ represent?