Because he painted 4 rooms and it takes 2 paint buckets to paint one room that is 8 paint buckets. Then you spilled 0.5 of a paint bucket plus the extra 1.5 of a paint bucket. Add 0.5 and 1.5 to get 2. Then add 2 and 8 to get 10. You bought 10 paint buckets!
Answer: 10 paint buckets