Abottle filled with water weighs 40lb. if the water by itself weighs 15 times as much as the bottle what is the weight of the bottle?

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.