You will need 1/8 cup of flour
STEP - BY - STEP EXPLANATION
What to find?
Cup of flour needed to make it 3/4 cup of flour.
Given:
Cup of flour need =3/4
Cup of flour already available = 5/8
To find the cup of flour needed to make it 3/4 cup of flour, we will follow the steps below:
Step 1
Let X be the cup needed more to make 3/4 cup.
Subtract 5/8 from 3/4.
x = 3/4 - 5/8
Step 2
Find the equivalent fraction of 3/4 such that its denominator is 8.
That is;

Step 3
Replace 3/4 by 6/8 in step 1 equation.

Step 4
Simplify the above.

Therefore, you will need 1/8 more cup of flour.