santas wrote:

is it not as simple as 0.9/1.2?

No. We shouldn't consider like that. We cannot simply add 0.1 for every cylinder. Actually we need to do add 10% of the sum.

Consider Power of one cylinder engine is 1. For every cylinder increase, power increases by 10%. So then

2 Cylinder Engine power = 1+(10% of 1) = 1.1

3 Cylinder Engine power = 1.1+(10% of 1.1) = 1.21

4 Cylinder Engine power = 1.21+(10% of 1.21) = 1.33

5 Cylinder Engine power = 1.33+(10% of 1.33) = 1.46

6 Cylinder Engine power = 1.46+(10% of 1.46) = 1.61

7 Cylinder Engine power = 1.61+(10% of 1.61) = 1.77

8 Cylinder Engine power = 1.77+(10% of 1.77) = 1.95

9 Cylinder Engine power = 1.95+(10% of 1.95) = 2.15

10 Cylinder Engine power = 2.15+(10% of 2.15) = 2.37

11 Cylinder Engine power = 2.37+(10% of 2.37) = 2.61

12 Cylinder Engine power = 2.61+(10% of 2.61) = 2.87

So

\frac{2.15}{2.87}=0.75 Hope this helps !