well,

If the 1's digit is 0, then 3*0+1 = 1, so the number is 10

If the 1's digit is 1, then 3*1+1 = 4, so the number is 41

If the 1's digit is 2, then 3*2+1 = 7, so the number is 72

If the 1's digit is 3, then 3*3+1 = 10, this is not a two digit number, so you are left with the first three cases.

Just check which one meets the criteria of "Subtracting 45 from the number reverses it"