31. The number of poplars planted by the students is expressed as a three-digit number, and the number of lindens as a two-digit number. In total, they planted 144 trees. If the extreme digits in these two numbers are swapped and the resulting numbers are added, the sum is 603. How many poplars and how many lindens were planted?
31. Since the sum of a three-digit and a two-digit number is 144, the hundreds digit of the three-digit number is 1. The sum of this digit and the tens digit of the two-digit number is 3, as the sum of the numbers written in reverse order is 603; therefore, the tens digit of the two-digit number is $2(3-1=2)$. The units digit of the desired three-digit number is $5(6-1=5)$, since the sum of the reversed numbers is 603, which means the units digit of the two-digit number is $9(144-1 * 5=29)$. Therefore, 115 poplars and 29 lindens were planted.