Explanation

Let the two-digit number be xy, where x is the tens digit and y is the units digit.
The sum of its digits is x + y.
The number is three times the sum of its digits, so we can write an equation: xy = 3(x + y).
Simplify the equation: 10x + y = 3x + 3y.
Rearrange the equation to get: 7x = 2y.
Now, if 45 is added to the number, the digits are reversed, so we can write another equation: xy + 45 = yx.
Simplify the equation: 10x + y + 45 = 10y + x.
Rearrange the equation to get: 9x = 9y - 45.
Divide both sides by 9 to get: x = y - 5.
Now we have two equations:

1. 7x = 2y

2. x = y - 5

Substitute the second equation into the first equation:
7(y - 5) = 2y
Expand and simplify:
7y - 35 = 2y
5y = 35
y = 7
Now that we have found y, substitute it back into one of the original equations to find x:
x = y - 5
x = 7 - 5
x = 2
So, the original number is 27.