[SOLVED] Question about some site's question/answer solution with given function

This is the problem on the other site, though I have a question about how/why they did what they did(for the solution).

**Here's the question:**

_____________________________________

**"**The projected sales volume of a video game cartridge is given by the function http://www.collegeboard.com/prod_ima...oice/SMC22.gif

where s is the number of cartridges sold, in thousands;

p is the price per cartridge, in dollars;

a is a constant.

If according to the projections, 100,000 cartridges are sold at $10 per cartridge, how many cartridges will be sold at $20 per cartridge?**"**

_____________________________________

So, I am guessing we're trying to find "s", but instead of using "10 = p", we're using "20 = p".

Now, about that function.

I assumed "s(p) = 3000 / 2p + a", plugged in would be look like this:

100(10) = 3000 / 2(10) + a

That's not what they did for the solution, they did this

100 = 3000 / 2(10) + a

They took out the (p) on left side of the function.

Here's their entire solution btw.

**Question:**

Why did they not include the (p) on the left side of the question, to make it "100(10) = 3000 / 2(10) + a" but instead did "100 = 3000 / 2(10) + a", when they gave the function http://www.collegeboard.com/prod_ima...oice/SMC22.gif?

Error or mistake?

Or is that how you're supposed to do it?