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.

Answer 1

If the number is ab,
b = a+7
10b+1 = 3(10a+b + a+b)

81 = 3(18 + 9)

Answer 2

u = t + 7

10u + t = 3(10t + u + t + u)
... = 33t + 6u

4u = 32t

solve the system by substitution or elimination