limiting factor problem

hi i am currently going through the kaplan ma2 book newest addition .
i have a problem that i cant work through and would appreciate any help given .
its one page 253 ch 14 decision making

example
at a production level of 8000 units per month ,which is 80% capacity ,the budget of export ltd company is as follows

—per unit$—8000 units $
sales————————-5.00———40,000
variable cost:
direct labour—————1.00———-8000
raw materials————–1.50———12000
variable overheads——-0.50———4000
fixed cost——————–3.00———24000
total cost———————4.50——–36000
budgeted profit————0.50———4000

an opportunity arises to export 1000 units per month at a price of $4 per unit
should that contract be accepted

a)spare capacity is available
b)increased profitability of 1000 (4000-3000 =1000)

c)if the limiting factor is materials ,budgeted contribution per $ of materials is
$20000 / 4000=$5 per $1
minimum price is therefore
materials/wages(as above)= $260
contribution$60 x 5 =$300
contract price = 560

i do not understand where the figure 20,000 and 4000 come from in part c.
why are they divided
why is contribution 60 x 5?

my solution for part c :

i know that lost contribution by accepting this job should be added to the price of the job
and in my mind it work out as
budget contribution/budgeted material = (4000+12000)/12000=4/3
contribution lost is = 4/3 x (1000 x 1.5) =2000
materials and wages = (3 x 1000) = 3000
hence contract price is 5000

please help i am very confused
thanks