Question 894804
THE INVESTMENT OF $8000 REGAINED.


n=number of lamps purchased.
n-4, number of lamps sold.
PROFIT, {{{p=100*(n-4)}}}.


Cost was {{{c=8000}}}.


{{{p=r-c}}}, profit is revenue minus cost.  Revenue is r.
{{{100(n-4)=r-8000}}}


Something is still missing.