Would You Help This Farmer ?

A farmer had 25 cows. To identify easily he numbered them from 1 to 25. Interestingly, each cow produces liters of milk equal to the respective number given. For example, a cow numbered 5 gives 5 liters of milk or a cow numbered 18 produces 18 liters of milk. Now that farmer had 5 sons. Farmer wants to share those 25 cows among his 5 sons with each son getting 5 cows. But he wants to make sure each son would get equal amount of milk.

How should he make 5 groups of cows to distribute among his sons?

Please, Help me!

This is what farmer need to do! 

Fair Distribution of Farmer's Cows

What was the interesting?

First let's name 5 sons as A,B,C,D & E. Now look at the image below.

Just collect numbers surrounded by same shapes. If you observe carefully answer, a difference of 4 created at one place which is compensated at other 4. Each son would be getting 65 liters of milk.

Fair Distribution of Cows