Some of these, I don't know how to answer. I can only give the answers to the ones I do know how to do.
16. 24 plants can be watered with this can.
Equation & Answer:
134.4 ÷ 5.6 = 24
17. 32 miniature bottles can be made from the large bottle of ketchup.
Equation & Answer:
198.4 ÷ 6.2 = 32
20. -3 + -8 = -11
-3 + 8 = 5
3 + -8 = -5
-9 + 4 = -5
-6 + -1 = -7
-8 + 9 = 1
Negatives plus negatives:
When there are two negative numbers together and the equation has + sign in it, the + sign changes to be a - sign.
21. 42 + -56 = -14
22. -49 + -17 = -66
Hope this helped in some way,
-Anime