还要劳烦royalex,,谢谢了，

A student responded to all of the 22
questions on a test and received a
score of 63.5. If the scores were
derived by adding 3.5 points for each
correct answer and deducting 1 point
for each incorrect answer, how many
questions did the student answer
incorrectly?

One-fifth of the light switches
produced by a certain factory are
defective. Four-fifths of the defective
switches are rejected and 1/20 of the
nondefective switches are rejected by
mistake. If all the switches not
rejected are sold, what percent of the
switches sold by the factory are
defective?

第一题，假设最后x道正确，那么错误的数量就是22-x

列出方程：
            3.5x-1*(22-x)=63.5

解出x=19，那么如题目所问，不正确的题目数就是22-x=3

第二题，由已知，所有产品中，

1/5产品有缺陷，其中4/5的产品被reject了，即此时，被reject了的产品有1/5*4/5，没有被reject的有1/5*1/5产品，

没有缺陷的产品中，1/20被reject了，即(1-1/5) * 1/20被reject了，

最后问如果没有被reject的都卖掉了，那么有缺陷的占所有卖掉的比例？

由上面的公式可以推出，卖出的产品中，有缺陷的有
            1/5*1/5 =1/25

所有卖出的总数应该是
            1-[1/5*4/5] -[(1-1/5) * 1/20] = 1/5

求比例就用
            ( 1/25 ) / ( 1/5 ) = 1/5

不知最后答案是否是我算得这个？

最后就是0.05