Importance should be given to all the informations that are already given in the question. based on those information's, the answer to the question can be easily determined.
The cost of planting 4 acres of corn = 520 dollars
Then
the cost of planting 1 acre of corn = (520/4) dollars
= 130 dollars
So
The cost of planting 3/5 acres of corn = (3/5) * 130 dollars
= 3 * 26 dollars
= 78 dollars
So the cost of planting 3/5 acre of corn is $78. I hope the procedure is clear enough for you to understand. You can always use this method in future for solving similar type of problems without requiring any help from outside.