I don't get it

I am grading a test. This was a complicated chapter so I wrote the work for the review on the board.

I told them to copy it.

I let them use the review sheet on their test.

One student wrote: "Sorry. I just haven't been able to concentrate a lot and I'm going to be honest: I have no idea how to do this. So I'll take the grade and just work harder after break"

OK.

There is no time for me to retest on this if I am to cover all of the material left by the end of the year.

BTW there were 2 numbers written out of 15 (with the work to support them) and they were wrong.

"I can accept failure. Everyone fails at something. But I can't accept not trying. It doesn't matter if you win as long as you give everything in your heart."
--Michael Jordan