Not sure if this is college math and what you're even asking, but if I'm understanding it right, it takes Eric 30 30 minutes. meaning 900 minutes. It takes his brother 20 20 minutes meaning 400 minutes. You're asking for the average time with them working together so 900+400=1300, 1300/2=650. It takes them 650minutes working together. 650 isn't a perfect square so I'd rather not expand on that part.
