Answer: 39
Explanation:
You exchange $20 for twenty bottles of mineral water.
[Drank: 20
Exchanged: 20/2=10]
[Drank: 20+10
Exchanged: 10/2=5]
[Drank: 20+10+5
Exchanged: 4/2=2]
[Drank: 20+10+5+2+1
Exchanged: 2/2=1]
Drank in total: 20+10+5+2+1+1
which is = 39