Let's denote the tens digit of the number as "x" and the units digit as "y". According to the given information:
The sum of the digits is 12:
x + y = 12
The units digit is 3 times the tens digit:
y = 3x
To find the number, we need to find the values of x and y that satisfy both equations simultaneously.
From equation (2), we can substitute the value of y in equation (1):
x + 3x = 12
4x = 12
x = 3
Substituting the value of x back into equation (2):
y = 3 * 3
y = 9
Therefore, the number is 39.