The percent $P$ of companies that are still in business $t$ years after the fifth year in operation can be represented by the equation
$$
P=-3 t+50
$$
a. What percentage of companies are still in business after 1 year in operation?
b. What percentage of companies are still in business after 25 years in operation?
c. After how many years are there only $35 \%$ of companies still in business?