Let x = The number of students majoring in math ONLY let y = The number of students majoring in computer science ONLY Let z - The number of students majoring in computer science.+ = + = + + = So we substitute the corresponding numbers or unknowns for the above three statements and we have this system of equations: x + 120 = 300 y + 120 = z x + 120 + y = 540 Solve that and get x = 180, y = 240, and z = 360 The answer asked for was z, which is 360. Edwin