Amanda pours 3/4 of her water into Barbara’s empty bottle. Barbara then pours 3/5 of her water into Charlie’s empty bottle. Charlie then pours 1/2 of his water into David’s empty bottle. If David now has 18 ounces of water, how many ounces does Amanda still have?

20 ounces of water

Step-by-step explanation:

Work backwards. David has 18 ounces, which is half of Charlie's. So, Charlie had 18•2=36 ounces. That was 3/5 of Barbara's water, so Barbara had 60 ounces. That is 3/4 of Amanda's water, so Amanda had 80 ounces. To find how much Amanda had left subtract 60 from 80 to get 20.