What number has the dame valuã© as 12 tens

In a two-digit number the units' digit is 7 more than the tens' digit. the number with digits reversed is three times as large as the sum of the original number and the two digits. find the number.

1) prove that 731^3−631^3 is divisible by 100 2) prove that 99^3−74^3 is divisible by 25

The difference of three and the reciprocal of a number is equal to the quotient of 20 and the number. find the number.