Economic Teacher got PMS?

The previous group ended their Economic tutorial early. So I went in early. There are a few students crowding around the lecturer.

When JY got in the room, it just happened that the teacher said, 'that is common sense' in a impatient tone.

After our tutorial, we had a doubt with 1 of the questions. We were struggling between asking and not asking that lecturer who doesn't seemed to be in good mood (not that she seemed to be ever in good mood). But we had to clarify the doubt no matter what so I decided go and ask her.

She only explain briefly. And when I try to explain to make sure what I understand is correct. Guess what? She just blurt out saying that is so simple to understand that even primary school kids know. (-.-)'''

What I asked is how to find out the burden of tax on producer and she just said take the total revenue minus the burden of tax on consumer. And she never said what revenue. I wanted to clarify that but after she said even primary school kids know, I decided to look at JY and see if she want to ask further which apparently she decided not to and she thanks the lecturer and we went off.

What may be common sense to her may not be common sense to us. We have got no background on Economics. We had never taken that before. How can she be like that?