We're told to use n for Nicole's age. Let's use j for Julia's age.
From the first sentence, j=1/2 n
From the 2nd sentence, j+1 = 3/4n. or j = 3/4 n - 1
We now have two equations which are equal to j, so using substitution, they are equal to one another.
1/2 n = 3/4 n - 1
Subtract 3/4 n from both sides
-1/4 n = -1
Multiply both sides by -4
n = 4
Nicole is 4 years old.
Julia is half of Nicole's age, so she Is 2 years old.