4. Person A and Person B agreed to meet at a restaurant for dinner. Due to the restaurant's popularity, when A arrived, they took a waiting number and waited for B. B arrived a while later but didn't see A, so they also took a waiting number. While waiting, B saw A, and they both showed their waiting numbers. They found that the digits of the two numbers were reverse sequences of each other, and the sum of the digits (for example, the sum of the digits of 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?
【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: $\left\{\begin{array}{l}\mathrm{B}-\mathrm{A}=2 \\ \mathrm{~B}+\mathrm{A}=8\end{array}\right.$
$B=(8+2) \div 2=5$
$\mathrm{A}=8-5=3$
Therefore, the number of A is 35.