3 times

For the first time, divide 27 pearls into 3 equal parts, take 2 of them and weigh them at both ends of the balance. If the balance is tilted, consider 9 light pearls; if not, consider 9 unweighted pearls. In the same way, divide nine pearls into three equal parts, weigh two at each end of the balance, and get three "suspicious" pearls again. Take out two scales. If the balance is skewed, the light one is defective ~ otherwise, it is not called defective.

20 days

Let the raw water of the reservoir be X, and discharge water A and B every day, according to the meaning of the question:

X=40(a-b)

X = 40 (1.1a-1.2b) (both are true)

So the solution is

X=20a

That is, it can only be stored for 20 days without adding water.