The official explanation given by Beestar.org is

The ratio between the red marbles and the white marbles was 3:1 at the beginning. After 24 red marbles were removed, the ratio was 9:4. So the number of removed red marbles represented (3-9/4) = 3/4 of the white marbles. Hence the number of white marbles was 24 ÷ (3/4) = 32.

这个方法看起来挺简单.但在美国的小学生中能想出此逻辑的很少(比我一开始想象的少得多).各位家长请如实地交流一下,你的孩子自己想出此解法了吗?据我对周围中国家长小孩的观察,我估计大概是这样:

Assuming the child is a fifth or sixth grader in US.

The child solved the problem correctly by himself (not guessing): top 5%

The child solved the problem correctly with some hint from parent: top 10%

The child could not solve the problem, but eventually understood the parent's thorough explanation and could apply it to similar problems: top 30%

The child mostly understood parent's explanation, but could not apply the logic to solve similar problems: top 50%

The child found the solution too hard to understand despite explanations: top 70%

Neither the child nor the parent could solve the problem: the rest.

我小学五年级时是班上的数学尖子.象这样的题见多了.做起来小菜一碟. 我想现在国内好小学的五年级学生应有一半能解这样的问题. 但在美国我们的孩子虽已在班上数一数二,可实际上与我们以前和现在国内的学生水平差得很远. 我们的孩子都很聪明, 但在学校学到的数学真是太少了. 如果他们象周围老美那样跟着学校上, 等走上将来更全球化的社会, 他们绝不是中国印度那些 sharp kids的对手. 我自己很忙,不可能整天教小孩数学,只能找几个好的program, 让孩子多参与. 尽力而为了.