THIS is where I am confused about triangle vector calculation

Okay so I have a thread posted about my confusion regarding calculating the area of a triangle.

Long story short, I was confused what to do with the cross product to get the area.

I`m going to take this example out of my book because it doesn`t make any sense to me.

We have three points:

P (2,2,0)

Q (-1,0,2)

R (0,4,3)

PQ (-3,-2,2)

PR (-2,2,3)

PR x PR (-10,5,-10)

I can do this myself, and this is what the book gives as answers aswell.

What really gets me going is I know, both from my teacher in class and this forum that I simply put the cross product into a formula and we get this:

(1/2) | square root of a^2 + b^2 + c^2 | (magnitude)

which is in this case (1/2) | square root of 100 + 25 + 100 |

I've always been solving this type of question like this, however... the book says the anser is 15/2. How did they get this? How come they did not find the magnitude and then divide it by half?