13.102. Given two two-digit numbers, the second of which is denoted by the same digits as the first but written in reverse order. The quotient of dividing the first number by the second is 1.75. The product of the first number and its tens digit is 3.5 times the second number. Find these numbers.
Solution.

Let's represent the first number as $10 x+y$, then the second number is $-10 y+x$.

According to the condition: $\left\{\begin{array}{l}\frac{10 x+y}{10 y+x}=1.75, \\ (10 x+y) x=3.5(10 y+x) .\end{array}\right.$ Solving the system, we find $x=2$, $y=1$. The required numbers are 21 and 12.

Answer: 21 and 12.