1. A shop sells milk in paper cartons. The price for 1 carton of milk is 5000 VND. To encourage the recycling and environment protection movement, the shop allows customers to exchange 5 empty cartons for 1 carton of milk. Lan is given 90000 VND by her mother to buy these cartons of milk. With that sum of money, after buying and exchanging, how many cartons of milk can Lan have altogether?



