A farmer who grows strawberries defines a workday yield as the

A farmer who grows strawberries defines a "workday yield" as the number of liters of strawberries that a worker is required to pick in one day. Based on this definition, how many workers must the farmer hire to pick the entire expected crop of strawberries in 15 calendar days?

(1) A workday yield is 45 liters
(2) The expected crop is 13,500 liters

There are two variables we need to figure out, to be able to figure out the number of workers required. How much a workday yield is, and what is the expected crop of strawberries.

Statement (1) is insufficient, we do not know the expected crop of strawberries required.
Statement (2) is insufficient, we do not know the workday yield.

Together, we have 13,500 expected, and a workday yield of 45 liters. The entire equation is number of workers * workday yield * 15 calendar days = expected crop. Substituting what has been provided, we have number of workers * 45 * 15 = 13,500.

We don't really need to figure this out, as the answer is probably going to be C. However, there are a few things I am unsure if GMAT will try to trick us with or not. Firstly, 45 * 15 MUST be <= 13,500. If 45 * 15 is > 13,500, then the number of workers would be <1, which... is not possible. Secondly, number of workers MUST be positive integers. So let's work it out to be sure. It takes about 15-20 seconds probably to figure out that 45 * 15 = 675, so definitely way less than 13,500. By the way, to quickly calculate whether 13,500 is fully divisible by 675, it's not too much of a hassle. Multiply 675 by 2, we get 1,350. Again, we don't have to work it out completely, just know that 13,500 is divisible by 1,350 because they share the same numbers, 13,500 just with a few more (actually just one more) 0 than 1,350. But yes, if we really want to figure it out, then we need 20 workers.

So when we do figure it all out, answer is definitely C.