难倒一片的小学数学题

It's posted above. You need to fill in the gaps with the digits from 1 to 9 so that the equation makes sense, following the order of operations - multiply first, then division, addition and subtraction last。
如上图所示。你需要用1到9的数字将所有空格填满，并且使等式成立，计算的顺序为，先乘，接着除，然后是加，最后是减。

There is no complicated maths involved, only basic arithmetic. But it’s not a walk in the park。
这题目没有涉及任何复杂的运算，只有基本的算术。但做起来一点也不容易。

“This problem is difficult even for adults good at math, so it will be difficult for students in grades 3, and even more challenging for students in the highlands,” teacher Tran Phuong told VN Express。
一位老师说：“就算对数学很好的成年人来说，这道题也很难解。所以对三年级的学生来说肯定很难，对高地地区的学生就更难了。”

He added: “I sent the problem for some people, including a doctorate in economics with mathematics, but they have not given the answer。”
他还说：“我把这道题发给了一些人，其中包括一位有数学背景的经济学博士，但他们都还没有给出答案。”

Vietnam does very well in the international PISA tables that compare 15-year-olds’ performance in maths, science and reading. Vietnam ranks 17 in maths and 8 in science, which outperforms many Western countries like the UK (26 and 20) and US (36 and 28)。
越南的学生在国际学生评估项目的数学、科学和阅读领域表现都不俗。他们在数学领域排名17，在科学领域排名第8，比英国(这两项分别排名26和20)和美国(分别排名36和28)都出色。

No wonder if they are given problems like this..。
难怪他们能出这样的题目....。

Can you do it?
你能解出这道题吗？

Here is one of the solutions from a commenter at Gizmodo: There are other possible solutions, but the first one I came up with was: 6, 9, 3, 5, 2, 1, 7, 8, 4 in that order ... How I got to it was assigning each blank to a letter and writing the whole thing as an equation then grouping like terms。
以下是Gizmodo网站一位网友给出的答案：可能还有别的解法，但我最先想到的是：空格中依次填6, 9, 3, 5, 2, 1, 7, 8, 4。我用字母代替每一个空格，然后把整个等式列出来，再进行同类项合并。

So... a+13(b/c)+d+12e-f+(gh/i)-21=66.
算式是这样的：a+13(b/c)+d+12e-f+(gh/i)-21=66。

Then you add 21 to both sides and a+d-f+13(b/c)+12e+(gh/i)=87. And since you’re limited to plugging in numbers between 1-9 for a-i, you see that there’s only so many combinations that will yield numbers close to 87. So then you start guessing and checking how large you have to make the various numbers to make it work. You need smaller numbers in the denominators and the subtraction and larger numbers being multiplied。
给算式两边都加21，就得出a+d-f+13(b/c)+12e+(gh/i)=87。字母a到i的空格中只能填入1到9 的数字，计算结果能得出87的变化组合也就那么多。所以你就可以边猜边调整，最后得出正好的结果。分母和减号后面的数字要小一些，乘号后面的数字要大一些。