Calculator Not Allowed.

Today we moved to maths (A and mental). For the first time we can remember, drawing an angle with a protractor was required and the mental maths paper included an algebra question.
Could you do this in 10 seconds?:
3k+n=? "k equals 10 and n equals eleven" said the voice, twice.
But perhaps you are not 10 or 11!

And actually, quite a few of them could!

I am very proud of how well they have done so far. Last test tomorrow. Calculator Is Allowed.