4. Person A and Person B agreed to have dinner at a restaurant. Since this restaurant is very popular, after A arrived, they took a waiting number and waited for B. B arrived at the restaurant a while later but didn't see A, so they also took a waiting number. While waiting, B saw A, and both of them showed their waiting numbers. They found that the digits of the two numbers were reverse two-digit numbers, and the sum of the digits (for example, the sum of the digits of the number 23 is $2+3=5$) of both numbers was 8, and B's number was 18 more than A's. What is A's number? $\qquad$
【Answer】 35
【Analysis】 Exam point: Position principle and sum-difference problems
Assume the two-digit number of A is: $\overline{\mathrm{AB}}$, the two-digit number of B is $\overline{\mathrm{AB}}$
Then: $\overline{\mathrm{BA}}-\overline{\mathrm{A}} \mathrm{B}=18, \mathrm{~B} \times 10+\mathrm{A}-10 \times \mathrm{A}-\mathrm{B}=9 \times \mathrm{B}-9 \times \mathrm{A}=9 \times(\mathrm{B}-\mathrm{A})=18$
Then: $\left\{\begin{array}{l}\mathrm{B}-\mathrm{A}=2 \\ \mathrm{~B}+\mathrm{A}=8\end{array}\right.$
$\mathrm{B}=(8+2) \div 2=5$
$\mathrm{A}=8-5=3$
Therefore, A's number is 35.