Missing Price

When I went to the store, I purchased four items. The following shows the cost of three of the items:

 $1.50
 $3.00
 $4.00

The line to the checkout was pretty long, so to quench my boredom I started playing with my pocket calculator while waiting.

I found out, to my surprise, that the four prices of the four items I purchased added to the same number as I got when I multiplied the four prices together.

What was the price of the fourth item?

Answer:

The three prices add to 1.50 + 3.00 + 4.00 = $8.50.
The three prices multiply to 1.50 x 3.00 x 4.00 = $18.00.
You might be able to see that adding another $0.50 will take the total to $9.00, and multiplying by another $0.50 will take the product to $9.00 also, but the answer can also be found using algebra. Let's call the unknown price P:

 1.50 + 3.00 + 4.00 + P = 1.50 x 3.00 x 4.00 x P
 8.50 + P = 18.00 x P
 8.50 = 18.00 x P - P
 8.50 = 17.00 x P
 8.50/17.00 = P
 P = 0.50.