The radius of a circle is 8 kilometers. What is the circle's area?

Nine more than four times a number is the same as one less than twice the number. find the number?

How can you construct perpendicular lines and prove theorems about perpendicular lines

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?