Luke borrowed some money from his friend in order to buy something. Luke agreed to pay back his friend $4 per week, and after 12 weeks, Luke still owed his friend $10. Write an equation for L, in terms of t, representing the amount of Luke owes his friend after t weeks.

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?