18. In the addition sum shown, each letter represents a different non-zero digit. What digit does $X$ represent?
A 1
B 3
C 5
D 7
E 9
18. D First note that when two numbers are added together the only possible carry from any column is 1 . Now, looking at the tens column of the sum, we see that $E+E$ leaves a total of $E$ in the column. Since $E$ is non-zero, the only way that this can happen is that there is a carry of 1 from the units column. So we have $1+E+E=10+E$, so $1+E=10$, that is $E=9$. Looking at the units column we see that $E+E=18$, so $S=8$ and there is a carry of 1 to the tens column. The addition sum may now be solved: $899+899=1798$. So $X=7$.