Can you solve the below tricky visual equation?

A man has eighty-one cows ( numbered 1,2,3...81 as such). The beauty is that cow no. 1 gives 1ltr of milk, cow no. 2 gives 2ltrs of milk and so on. The man wants to equally distribute the cows among his nine sons so that each one of them gets the same quantity of milk.

Take 9 from 6, 10 from 9, 50 from 40, and leave 6.

How Come ??

Why is the longest human nose just 11 inches?

A Doctor always sneezes before the rain. Doctor sneezes, Will it rain?

What number when divided in half becomes zero?

A mother is 21 years older than her child. In exactly 6 years from now, the mother will be exactly 5 times as old as the child.
Where's the father?

What has a ring but no finger?

There's a day, that comes around once a year.
To celebrate the man, you hold dear.
He gave you life, love and will pick you up if you fall.
When it comes to guys like him, He's the best of all.
Who is he?

How many bananas can you eat if your stomach is empty?