1. let x = the unknown number
60 - 3x = 15
45 = 3x
x = 15
So, the number is 15.
2. Let 1st number = x, 2nd number = (x + 2), 3rd number = (x + 4)
x + (x + 2) + (x + 4) = 105
3x + 6 = 105
3x = 99
x = 33
The three numbers are 33, 35, and 37