Question on high low with fall in variable cost

Number of units produced Total cost($)
4000 60000
10000 108000
13000 123000
17000 157000
24000 195000

Management is aware that variable costs decrease by $1.50 per unit if the number of units produced exceeds 20,000.

What are the variable costs per unit if production is less than 20,000 units?

Anwer. Highest activity level is 24,000 and lowest activity level is 4,000

Adjusted highest activity level cost = $195,000 + (24,000 × 1.50) = $231,000

Variable cost per unit at lowest level = ($231,000 – $60,000)/(24,000 – 4,000) = $8.55

Available Answers
8.55 (1 Mark)

question says the vc fall per unit after 20000 units so VC should not this be calculated by adding 1.50$ to the 4000 units above 20000u and then adding it to the total cost of 195000 to calculate VC per unit as if the Fall in VC never happened. This will give us VC per unit below 20000units.