Answer:
20
Step-by-step explanation:
20 is the least common denominator
There are 3 ways of solving a simultaneous problem, substitution method, elimination method and Gauss-Jordan method. I'm gonna use the substitution method since it's easier and i think it would suit your level more.
First let's try solving for y since it's easier to start with.
Firstly we have to find an equation for x:

Great, now we can use the substitution method to find the value of y using the first equation:

Now we know that y=1 we can solve the first equation we made:

And the answer is 
Double check:

And that's our final answer! (4,1)
Ok. So, if 8 pens cost £12. Then we would divide £12 by 8. Each pen costs £1.5.
So now we are going to multiply £1.5 and 10. £1.5*10=15.
The total of 10 pens is £15.
I hope this helps, if you need any additional help, just ask. I am here to help you as needed.
The answer is C . -7r^2 + 12r -8