Today, John has to transfer 50 euro to the bank account of a Dutch friend. He has written down the account number on a piece paper. But since he had forgotten to take out the paper from his trousers when he put them in the washing machine, one digit of the bank account number became unreadable. The note says: 3170?4847. The friend of John is climbing the Mount Everest at the moment, so it is impossible for John to call his friend. Suddenly he remembers that a for a valid Dutch bank account number it holds that the first digit times 9 + the second digit times 8 + the third digit times 7 + ...... + the nineth digit times 1 should be divisible by 11. John thinks for a moment and finds the correct number. What is it?

Best Bus, a bus company from Austin Texas would like to acquire a new bus. Somebody working at Best Bus found a nice second hand bus at Ebay (see the picture) for a reasonable price. When the president of the company sees the picture: "Yes man, this is a nice good looking bus, and pretty cheap too, but think man I never can satisfy my customers with such a bus." Why not?

At a particular ice cream van two types of ice creams are being sold. The small one costs 1 dollar, the big one 2 dollars. Marsha puts an amount of 2 dollars one the counter and gets without asking the big ice cream. Martha also would like to have an ice cream. She also puts an amount of 2 dollars on the counter. Then the ice cream seller asks her which ice cream she would like to have. Assuming that the persons do not know each other, how can you explain their behavior.