If 1 cup of milk is added to a 3-cup mixture that is 2/5 flour and 3/5 milk, what percent of the 4-cup mixture of milk?

This is the concept of algebra, the percentage of Milk in the 4 cup mixture will be found as follows;
the ratio of milk to mixture after 1 cup was added to 3-cups was:
1/3
the total ratio of milk in the mixture was:
1/3+3/5
=14/15
thus the percentage of milk will be:
14/15*100
=93 1/3%

Ais the answer for your question