Ineed with number 17 and 18. the fractions and decimals throw me off.

For number 17 changed the 13 3/8 into a decimal to help you. 3/4 would be .75 and 13 3/8 would be 13.375 
13.374 +.75 = 14 1/8 which would be 14. 125 
14.125 + .75= 14 7/8 which would be 14. 875 
so you can carry two more water bottles

The answer to number 17 is 2 water bottles. First you have to figure out how much pounds you have left to 15, thats when you subtract 15- 13 3/8= 1 5/8
Next, you subtract 3/4 from 1 5/8. You are left with 1/8 after subtracting 3/4 from 1 5/8 2 times. You are left with 1/8. You can not subtract 3/4 anymore, in this case if add another matter bottle, it will exceed the limit of 15 pounds. Hope this helped.