Let's call the number of $20 toys a. Since the store sold a total of 20 toys, the number of $40 toys is 20 - a. Thus, the total price of all the toys sold is:
20a + 40(20 - a)
20a + 800 - 40a
800 - 20a

Statement (1) is sufficient. If you know the prices of the two types of toys and you know the weighted average price of the toys sold, you have enough information to find the ratio of the number of types of toys sold. On the test, you don't need to solve for the exact amount, but here's how you would. We already know that the total price of all the toys sold is 800 - 20a, and this tells us that the total price of the 20 toys is 35(20) = 700. Thus:
800 - 20a = 700
100 = 20a
a = 5

Statement (2) is insufficient. We know that the total price of the toys is 800 - 20a, and that a must be an integer. If a = 3, 800 - 20a = 740, which is within the given range. If a = 4, 800 - 20a = 720, also within the given range. There are other possibilities, but two is enough: We don't know how many $20 toys the store sold. Choice (A) is correct.