In algebra a word that means something has been added

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?

What is the simplified form of square root of 400 to the 100th power ?

y=2x−1 5x−4y=1 is (1,1)(1,1) a solution of the system?