Cost function is $c(q)=q$
Demand function is $q(p) = 100/p$ for $p\le 20$. And $q(p)=0$ for $p>20$.
In order to calculate profit maximizing output and price
$$max [p(q)*q-q]$$
And marginal revenue = marginal cost for FOC.
But I cannot get proper result. Why? Please help me to calculate this or give me a hint. Thanks.