ROI and priority

Dear tutor, i found below question from kaplan exam kit.

Question:

The following information about the division of Grate Co is available as follows:

Risk environment Drvision X (low)

Revenue: $50,000
Operating cost: 20,500
Fixed cost: 11,000
Profit: 18500

Capital empoyed: 150,000

Half of fied cost for the division is allocated head office expenses. Grate Co has a cost of capital of 15%

In order to try and improve the current performance, the manager of division X is considering 3 newprojects.

capital investment profit
project 1 48,000 12,000
project 2 100,000 22,000
project 3 50,000 9,000

If the projects are ranked to their ROI, which project will be the priority choice for Division X?

The answer given is B project 2. I really don’t understand the workings given in the book. Please help me.

Below is the answer given:

ROI for project 1,2, 3 is 25%, 22% and 18% respectively. (calculate the ROI is not a problem for me)

Project 1+2

12000-48000 x > 22000-100,000x
52,000x>10,000
x>0.192

Project 2+3
22,000-100,000x> 9,000-50,000x
13,000>50,000x
0.26>x

May I know why need to calculate the project 1+2 and 2+3? and what are the funtions for?

Thanks a lot:)