Well.....the fact that he bought 200 yards is irrelevant in this problem.
BUT the amount he paid for them IS relevant.....
He paid 30
He sold for 40 sooooooo his profit was ?
Profit = sold - paid = 40 - 30 = 10
DO you know how to get a percentage now?
percentage profit is profit/paid x 100
=10/30 x 100 33.33%