Ineed with both questions :
part a:
a coffee shop uses 4 l of milk every day. if there are 15 l of milk in the fridge, after how many days will more milk need to be purchased? explain how you know

part b:
if only half as much milk is used each day, after how many days will more milk need to be purchased?

Part A: After 3.75 days more milk will need to be purchased. I know this because 4 can only go into 15 3.75 times.

Part B: If half as much milk is used each day after 7.5 days more milk will need to be purchased. I know this because 2 can go into 15 7.5 time.

Part A: The coffee shop can go 3 days until they need to purchase more milk. If you divide 15 by 4 you get 3.75. So, you wouldn't have enough milk to go through another day. You can also count by four until you get to 12. You wouldn't be able to go any higher.

Part B: The coffee shop will now be able to a week until buying milk. If you divide 15 by 2 you get 7.5. You can also count by two until you get to 14. 

Hope this helps! :)