High/Low Method

Hello Sir Moffat!

I have watched your F2 lecture on The High/Low method but was confused when you said let’s check the low month which was

200 units x $100(variable cost) = 20,000+10,000 (fixed cost) = 30,000

I understand the rules of working out the high/low method in the lecture, but would of thought the working out of the Low month would be reached by using the same method we use for the high, for example I would of worked it out like this

30,000 (total cost at low level) – 200 (total units at low level) x 100 (total variable cost per unit)

Which is obviously not correct!

Am I right in thinking your explanation of working out the low month can only be reached after working out the total fixed cost??

I hope I am making sense!
Please can you kindly explain ?

Many thanks