Question 1 in chapter 4 in F2 lecture

the question was as follows:

levels of activity (units) Total Cost ( $)
16.000 135.000
22.000 170.000

Variable cost per unit remain constant within the range of activity levels, but there is a step up of 5.000$ in the total fixed cost when the activity level exceeds of 17.500 units.

what is the total cost at activity of 20.000 unit by using high – loe method.

1. in my point:

variable cost per unit = ($170.000 – $135.000) / (22.000 – 16.000) = 5.8 $
the total cost at high levels was: 170.000 $
the total variable cost was : 5.8$ * 22.000 = 127.600$
the total fixed cost was: $170.000 – $ 127.600 = $ 42.400

At output of 20.000:
Due to variable cost per unit was unchanged and the total fixed cost also was unchanged
so:

the total variable cost = 5.8$ * 20.000 = $ 116.000
the total stepped fixed cost = $ 42.400 + $ 5.000 = $ 47.400
==> the total cost must was : $116.000 + $47.400 = $ 163.400

However, the correct answer in F2 notes was $163.000. I saw the solution in note but still did’t get why the minus $5.000 in total cost when determine variable cost per unit, not plus $ 5.000 into fixed cost at output 20.000.

please explain my confused with many thanks