If each unit has a variable cost of £6 then 23900 units will have variable costs of £143,400 Take this cost away from the full cost for 23900 units (£324,400 - £143,400) will give you £181,000 for the fixed costs. The same works for the 19000 units (19000 x 6 = £114,000). The full cost for 19000 units is £295,000 less the variable costs £114,000 = £181,000.
Comments
23900u - £324400
19000u - £295000
4900u - £29400
1u - £6 variable
fixed costs: £181k
@ 21000u FC £181k VC £126k
@ 36000u FC £193k (£181k + £12k) VC £205k (25000 @ £6, 11000 @ £5)
Hope that helps.