inversly means xy=k
lets say number of kameras is x
cost is y
if we have
15000(80)=k
1200000=k=xy
therefor if you wanted to make 250 cameras then subsitute
250 times y=1200000
divide both sides by 250
y=4800
it would cost 4800 per camera if you made only 250 cameras
use xy=1,200,000 and solve