An oil company discovered an oil reserve of 100 million barrels. For time $t>0,$ in years, the company's extraction plan is a linear declining function of time as follows:
$$
q(t)=a-b t
$$
where $q(t)$ is the rate of extraction of oil in millions of barrels per year at time $t$ and $b=0.1$ and $a=10$
(a) How long does it take to exhaust the entire reserve?
(b) The oil price is a constant $\$ 20$ per barrel, the extraction cost per barrel is a constant $\$ 10$, and the market interest rate is $10 \%$ per year, compounded continuously. What is the present value of the company's profit?