Can you replace the question mark with the correct number?

Solve the rebus riddle in the picture below.

A boy purchased a book from a bookkeeper and gave him $100.
The cost of the book is $50 but the bookkeeper has no change, so he gets the change from the next shop and returns the boy his $50.

After some time the next shopkeeper came with the $100 note and told the bookkeeper that the note was a fraud, so he took the money back.

How much loss did the bookkeeper face?

If I remove one from eleven it becomes Ten and if I remove one from nine also becomes Ten. How?

Two in a corner, one in a room, zero in a house, but one in a shelter. What is it?

Make the seating arrangement of 10 children in such a way that there are 5 rows with 4 children in each row.

Three ants are sitting at the three corners of an equilateral triangle. Each ant starts randomly picks a direction and starts to move along the edge of the triangle. What is the probability that none of the ants collide?

What goes up but never comes down?

What am I?

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?