The way to solve this is through guess and check.
40 - 1 = 39
1 * 15 = 15
If the answer were 1, the bottle would weigh 16 pounds
40 - 2 = 38
2 * 15 = 30
If the answer were 2, then the bottle would weigh 32 pounds
40 - 2.5 = 37.5
2.5 * 15 = 37.5
The bottle itself weighs 2.5 pounds
Hope this helps.
