7. A number consists of two digits whose sum is 12. If 18 is added to the number, the digits are

reversed. Find the number.

Let the digit at the tens place be x and the digit at the unit place be y. 

x+y=12  .........(1)

Required Number = (10x+y).

Number obtained on reversing the digits = (10y+x).

According to the condition, we get

(10y+x)−(10x+y)=18

→ 9y−9x=18

→   y−x=2     ..........(2)

Adding (1) and (2), we get

2y=14 →y=7

 → x= 12 - 7 = 5

Hence, the required number is 57.