Pricing High-Low Method

Hi John. The first Section B question I looked at in Kaplan was on the High-Low method which I don’t think you covered. I tried following it backwards and understood what was done with the material & labour costs, it was simply taking the lowest output and dividing each of the costs by the units which gave you the cost PU. I don’t however know why the lowest output was selected nor do I understand where the variable cost PU of $3 came from. The question I refer to is in Kaplan Exam Kit valid Sep19, Dec19, Mar20 & Jun20 and is question 249 ALG Co. If you do not cover the high-low method perhaps you could point me in the right direction?
Thanks John,
Edward McGarry