I'm assuming that 4 3/12 is a mixed fraction:
So with your equation: 4 3/12 + n = 5, I would first turn 4 3/12 into an improper fraction then solve.
12*4+3 = 51 is the new numerator so the equation is:
51/12 + n = 5
Now subtract 51/12 from both sides:
n = 5 - 51/12
Now find a common denominator in order to subtract the two numbers on right:
n = 60/12 - 51/12
n = 9/12
n = 3/4