If the customer buys 4 pencils he will get the 5th one free.
This means, in every 5 pencils, he is getting one free pencil. 60 pencils can be split into 12 groups of 5 pencils each. In each group he is getting 1 free pencil, so in 12 groups, the customer will get 12 free pencils.
So, we can say that if the customer buys 60 pencils on sale, he will get 12 of them free.
