Answer:
D. for the first one, A. for the second (not certain)
Explanation:
1. 36/2 (two books every visit) = 18, multiplied by 3 because that's how many often he does to the library (every 3 weeks)
2. 13/2 = w/5 because for every 13 cups of water, there is two cups of cleaner, so for w (unknown amount of water) there would be 5 cups of concontrated cleaner.