Question 765251
This question is probably faulty; not because of the variable number of worker and days as part of the rate, but because of ,"2z work at the same rate..." .


The quantity of workers, 2z, will work at a different rate than 5x workers, and at a different rate than just one worker.  


I will give a solution, but it may be to a question different than the one you really want.


Rate information given is that 5x workers do this way:
5x workers____________4y days
This means,
1 worker_____________{{{5x*4y}}} days, or {{{20xy}}} days
Rate of 1 worker is {{{1/(20xy)}}} car per day.


Now for a group of 2z workers, the rates of all the workers in the team or group are simply additive.  This gives us an expected rate for these 2z workers of {{{2z(1/(20xy))=(2z)/(20xy)=highlight(z/(10xy))}}} car per day.


That rate as is obvious is in car per day; and you wanted the number of days per car, which is the reciprocal, so ....
{{{highlight((10xy)/z)}}}  DAYS PER CAR.