Billy sells twice as many $20 tickets as Tim, and Tim sells three times as many $10 tickets as Billy. How many tickets did Billy sell? Tickets are either $10 or $20.

(1) Tim sold a total of 35 tickets

(2) Together Billy and Tim sold 70 tickets for $1000

条件(1)是不足够的。设x为Tim所卖的售价为$20 的票数，y为Billy所卖的售价为$10 的票数。那么

Billy 卖的票数共为：2x ($20 tickets) + y ($10 tickets)
Tim 卖的票数共为： x ($20 tickets) + 3y ($10 tickets)

由条件(2)可以得出 70 = x + 2x + y + 3y 和1000 = 20(x + 2x) + 10 (y + 3y)这两个方程。我们将其组成一个二元一次方程组，解这个方程组可得y = 10 和x = 10, 也就是说Billy 总共卖了 2(10) + 10 = 30 张票.

这里的陷阱是条件(1)完全没有必要，因为光条件(2)就能得出Billy所卖的票数。条件(1)或许可以帮助回答问题，但没有必要，仅靠条件(2)就足够了。