Sorry I can't type Chinese in this computer.
I think my little son (7 and half years old, finished 1st grade) can do the question you mentioned here, but I never really teached him this fraction myself, somehow he could figure it out. He give me a lot of surprises but he is also reluctant to remember the time table. My little son is not a super fast thinking kid, but he almost always figure out the way to solve a problem.
Give an example, we went to a IMACS open house, the teacher ask a question like this:
She has a bag of plastics pieces, there are two sizes: big and small, three colors: blue, red and yellow, 4 shapes: square, triangle, circle and oval. How many pieces she has in the bag. All the kids could not figure out, but my son gave the right answer: 24. I was really surprised with his answer, so were the other two moms sat beside me, because he never learned this. After the open house was finished, I asked him how he get the number, he said, each size has 12 (3 color, 4 shape make 12), so two different sizes pieces add to 24.
So it is not necessary the kids can solve this problem are already taught by their parents. Some kids can just do it.
