Funny topic of foreigners buying pens

The price of a pen is X yuan, and the price of a pencil is Y yuan.

Naughty is 2x+y=20.

Smile is x+2y= 13.

Therefore, two smiles are 2x+4y=26.

Therefore, 2x+4y-(2x+y) = 2x-2x+4y-y = 26-20 = 6.

So, 3y=6.

So, y=2

And because 2x+y=20, y=2.

Therefore, 2x=20-y=20-2= 18.

So, x=9.

So the price of pens is 9 yuan, and the price of pencils is 2 yuan.

But you can't learn quadratic equation in the third grade of primary school, but it seems that other methods can't solve this problem.

In fact, I think this question is not about the ability to solve problems, but about the ability of children to question authority and admit their own shortcomings, because in the eyes of children, teachers are authority and truth, and ordinary children dare not question teachers at the knowledge level. This problem is obviously beyond the knowledge of a third-grade child. Unless the child has learned the Olympic Mathematics, the teacher may just want to see if anyone dares to tell the teacher about it.