I don't understand how to do this.
Remember that the sum of the probabilities = 1
So we have
n + 1/3 + 1/8 + 1/6 = 1
n + 8/24 + 3/24 + 4/24 = 1
n + [ 8 + 3 + 4 ] / 24 = 1
n + [ 15/24] = 1
n + 5/8 = 8/8 subtract 5/8 from both sides
n = 3/8
