There are 17 trees along the road from Basil's home to a swimming pool. On his way way to and from a swim Basil marks some trees with a red stripe. On his way to the pool he marks the first, the third, the fifth tree and so on. On his way back again, he marks the first tree he comes to, the fourth tree, the seventh tree and so on, missing out two trees each time. By the time he gets home how many trees have no mark?
