Problem:
Arthur is a an 12 year old with an IQ of 150. On his birthday, Arthur received a gift of $500 in cash from his Aunt, who told him to buy what he wanted. Since Arthur was interested in stamps, he ordered the "100 Special Stamps a Month" offer for 12 months from the Excelsior Stamp and Coin Company. Arthur was mistaken about the price of the "100 Special Stamps a Month" offer. It was actually $50 a month. Arthur thought it was $5 dollars a month, and sent Excelsior $60, which he thought was full payment. After Arthur sent the $60, he lost all interest in the stamps and ignored all further requests for payment Excelsior.
Excelsior, outraged over Arthur's delinquent account, has sued Arthur's parents for $540, representing the balance due. Who wins and why?