How many halves are in three wholes

What are the equivalent ratios for 24/2= /3= /5.5=108/ = /15

Each person in an it department of 25 people works on at least one of 3 major projects: system security, program implementation, platform development. 10 people work on system security, 13 work on program implementation, and 12 work on platform development. 4 people work on system security and platform development, 5 people work on program implementation and platform development, and 2 people work on all three projects. how many people work on system security and program implementation?

Hi i’m not sure how to do question 20 if u could explain how to do it that’d b great