A sandwich shop has 4 different types of sandwiches: ham (H), turkey (T), chicken (C), or PB&J (P). And each sandwich can be ordered with white bread (w),
multi-grain bread (M), or rye bread (R).

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?