16. The pedestrian named to the traffic inspector the number of the car whose driver had grossly violated the traffic rules. This number is expressed as a four-digit number, the unit digit of which is the same as the tens digit, and the hundreds digit is the same as the thousands digit. Moreover, this number is a perfect square. What is this number?
16. Since the square of a number can end in the digits $0,1,4,9,5$ or 6, the last two digits of the desired number can be $00,11,44,99,55$ or 66. This four-digit number is the square of a two-digit number, the digits of which are the same. Such a two-digit number cannot be 11 or 22, since their squares are three-digit numbers. It is not difficult to find among the seven other numbers that the specified requirements are met by $88\left(88^{2}=7744\right)$.