how many bags will he need?

The area of the lawn is (15 feet)x(40 feet) = 600 square feet

If the seed is only sold in 5 lbs bags, and each pound covers 100 sq feet, then one 5 pound bag covers an area = (100sq ft per lbs)x(5 lbs) = 500 sq feet

So you'll need two 5 lbs bags, but will use only one pound out of the second bag. Ideally, get one 5 lbs bag and one 1 lbs bag.