You have four resistors, each of which has a resistance r. it is possible to connect these four together so that the equivalent resistance of the combination is also r. how many ways can you do it? there is more than one way.

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?