A and B went to market with 30 pigs each. A sold his pigs at 2 for $1, and B sold his pigs at the rate of 3 for $1, and they, together, received $25. The next day A went to market alone with 60 pigs, and, wishing to sell at the same rate, sold them 5 for $2, and received only $24. Why should he not receive as much as when B owned half of the pigs?