When you see ratio, think fraction. So, we want :
William's money / Michael's money
They tell us that William has $280 and Michael has $420. Thus,
280 / 420
Now we need to simplify. both 280 and 420 share a factor of 140. Thus,
280/420 = (140 * 2) / (140 * 3) = 140/140 * 2/3 = 1 * 2/3 = 2/3