On a test that has a normal distribution, a score of 84 falls three standard deviations above the mean, and a score of 60 falls one standard deviation below the mean. Determine the mean of this test.

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?