A certain number of two digits is three times the sum of its digits and if 45 be added to it, then digits are reversed. The number is?

Let unit digit = x and ten's digit = y
3(x + y)= 10y + x .....(i)
10y + x + 45 = 10x + y ....(ii)
2x - 7y = 0.9x -9y = 45
or x - y = 5 .... (iii)
Solving these equations, we get
x=7, y=2
∴ Required number = 27