Imagine you’re driving a bus. At the fist bus stop, 13 persons enter the bus. At the second, 7 leave and 5 enter. At the third, 4 leave and 6 enter. At the third, 3 leave and 8 enter. At the forth, 2 leave and 5 enter. At the fifth, 13 leave and 2 enter. At the sixth, nobody leave, but 4 enter. At the seventh, 6 leave and 8 enter. At the eights, everyone leave. How old is the bus driver?