Answer:

Step-by-step explanation:
We can see the fractions
and
of cups.
It can be seen that denominator has 4 i.e. the fraction
.
Let us suppose, a unit is equal to
of a cup.
Susan was supposed to use
of a cup.
i.e. 5 units of butter was to be used.
But, actual recipe has only 3 units of butter.

a. 
Alternatively, we could have directly divided the given fractions:
