25 cakes can be completely decorated using 4 gallons of icing.
Explanation:
Given:
4 gallons of icing
16 cups =1 gallon
No. of icing cups =
cups
=64 cups
No. of cups required to decorate and frost a cake= 4% of total no. of icing cups
=

= 2.56
2.56 cups of icing is required to decorate each cake.
Maximum no. of cakes decorated =

= 25
25 cakes can be completely decorated using 4 gallons of icing.