儿子昨天带回来一张数学作业，老爸一看，就知道是为州里每年一度的统考准备的。十道题，全是一个模式：X只昆虫加蜘蛛，Y条腿，多少只昆虫，多少只蜘蛛？
老爸问儿子：“Did your teacher show you how to do this?”
儿子回答：“No, dad. We ran out of time in the class. I guessed some of them.”
老爸一看，就知道儿子是瞎猜的。也难怪儿子，当年姐姐这个年纪从州里统考回来，就是这道题不会解，回来问老爸呢。
老爸其实也不知道如何讲解才能让三年级孩子理解。当年给姐姐讲解，是用列表的办法。５只昆虫和蜘蛛，共有３２条腿，老爸想了半个小时，就教女儿列一个table: 0只昆虫，５只蜘蛛，４０条腿；１只昆虫，４只蜘蛛，３８条腿；。。。女儿当年是懂了。９年后，老爸又面对同样的问题，还是同一种解法嘛？儿子的十道题，最高的数字是１２只昆虫加蜘蛛，不行啊！
边作晚饭边想办法。半个小时，菜饭上桌，老爸终于有了解法，可就不知道儿子是不是能理解了。
“See, this one, 8 spiders plus insects, 60 legs. We assume all are insects. 8 insect, there are 48 legs. 60-48=12. So we are 12 legs less than the 60 legs. Each time we switch one insect to a spider, we increase 2 more legs. so, we need to switch 6 insects to spiders to get 12 legs more. so it should be 6 spiders and 2 insects.“ 老爸看着儿子，希望儿子能开窍，比女儿聪敏些。
“I don't get it, dad.”儿子看着老爸发楞呢。
“Ok, you do it with a pencil and paper. 8 insects plus spiders. Assume all 8 animals are insects, how many legs?”老爸死心了，知道儿子不是天才。
“6,12, 18, 24....48 legs!”儿子扳指头作乘法还是挺在行的。
“Great! So 48 legs, it is 12 less than the 60 legs, right?”老爸耐心地问。
“Yes.”
“Then, we have to increase spider number to get more legs. When you change one insect to a spider, we get 2 more legs. so for 12 more legs, how many spiders you need?”
“That's easy, 6 more spiders.”儿子回答。
“Then, it is 6 spiders, and 2 insects. Let's see whether the anser is right. 6 spiders, 48 legs, and 2 insects, 12 legs. So the total legs are 60, matching the answer. Correct!”
“Oh, I get it! Let me finish the rest.” 儿子边吃边解题，十几分钟都解出来了。
嘿嗨，老爸没有天才基因，儿女如何能成天才？不过，笨老爸还是可以慢慢改进自己的教育方法的。
（老爸今天想引进代数的概念(X)，儿子是“I don;t know what you are talking about, dad!”。老爸彻底死心了。）
（１０／２２／２００８）
