13. The tens digit of a two-digit number is three more than the units digit. When this two-digit number is divided by the sum of its digits, the answer is 7 remainder 3 . What is the sum of the digits of the two-digit number?
A 5
B 7
C 9
D 11
E 13
13. B Let the units digit of the number be $x$. Hence the tens digit of the number is $x+3$ and the sum of the digits of the number is $2 x+3$. The information in the question tells us that $10(x+3)+x=7(2 x+3)+3$. Hence $11 x+30=14 x+24$ which has solution $x=2$. Therefore the sum of the digits of the two-digit number is $2 \times 2+3=7$.