Why is it very easy to weigh a fish?

There was once a college that offered a class on probability applied to the real world. The class was relatively easy, but there was a catch. There were no homework assignments or tests, but there was a final exam that would have only one question on it. When everyone received the test paper it was a blank sheet of paper with a solitary question on it: 'What is the risk?'.Most students were able to pass, but only one student received 100% for the class! Even stranger was that he only wrote down one word!
What did he write?

There are 3 apples in the basket and you take away 2. How many apples do you have now?

Forwards I am heavy, backward I am not. What am I?

What is both possible and impossible at the same time?

When we see 2 but call 10?

What seven-letter word has hundreds of letters in it?

A cat had three kittens: Snophy, Topsy and Jorden. What was the mother's name?

What can you hold in your right hand, but never in your left hand?

A bank customer had $100 in his account. He then made 6 withdrawals. He kept a record of these withdrawals, and the balance remaining in the account, as follows:

Withdrawals Balance left
$50 $50
$25 $25
$10 $15
$8 $7
$5 $2
$2 $0
----- -----
$100 $99

Why are the Totals not tallying?