Problem 456

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.