9.- Three friends in difficulties share a cup of coffee that costs 30 cents, so each one of them puts 10 in.

When paying they ask for a discount and the owner reduces the price in 5 cents so each one of them takes one cent and keep 2 in a common fund.

Later they do the numbers and they say:

Each one has paid 9 cents so we have spent 9x3=27 cents, plus the 2 in the fund equals 29.

Where is the missing cent?