Solve 6×1/5 solve the equation

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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?

If t17 = 3 (t5) in an arithmetic progression, find t1 in terms of d.

What multiplys to be 1 but adds up to be 4