Mistake in OpenTuition course notes (Chapter 5) or … ?

In the answer for Example 2 in course notes https://opentuition.com/acca/f5/acca-f5-notes/
there is a strange way of profit calculation.
Optimal production plan there: 20000 units A and 8000 units B.
Then we need to calculate the profit:

Profit
$
A: 20,000 × $17 340,000
B: 8,000 × $8 64,000
404,000

less “fixed” costs:
[A: 20,000 × $15
B: 10,000 × $6] 360,000

Profit $44,000

Question is – why fixed costs applied to 10000 units B? It looks like apparent mistake, but maybe I don’t understand something.