Celyna bought 300 grams of candy A for $\$ 5.00$, and $x$ grams of candy B for $\$ 7.00$. She calculated that the average price of all of the candy that she purchased was $\$ 1.50$ per 100 grams. What is the value of $x$ ?
(A) 525
(B) 600
(C) 500
(D) 450
(E) 900
Celyna spent $\$ 5.00$ on candy A and $\$ 7.00$ on candy B, or $\$ 12.00$ in total.

The average price of all the candy that she purchased was $\$ 1.50$ per 100 grams.

This means that if Celyna bought 100 grams of candy, she would have spent $\$ 1.50$.

If she bought 200 grams of candy, she would have spent $2 \times \$ 1.50=\$ 3.00$.

How many grams of candy would Celyna need to buy to spend $\$ 12.00$ ?

Since $8 \times \$ 1.50=\$ 12.00$ (or $\$ 12.00 \div \$ 1.50=8$ ), then she would need to buy a total of 800 grams of candy.

Celyna bought 300 grams of candy A, and so she must have purchased $800-300=500$ grams of candy B. The value of $x$ is 500 .

ANsWER: (C)