I need help solving these three calculus questions

1. A rectangular area along the bank of a straight section of a river is to be enclosed with 280 feet of fencing. Only three sides need to be fenced since the river will be bound on the other side. What are the dimensions that will produce the greatest enclosed area? ; Epidemiologists have found a new communicable disease running rampant in Knoxville, TN. They estimate that t days after the disease is rst observed in the community, the percent of the population infected is approximated by m5" = :4 1000 1 Bit) for 0 S t S 20; What is the maximum percent of the population infected? 3. The manager of an apartment complex knows from experience that he can rent out all 100 units if the rent is $800 per month. A market survey suggests that, on average, one less unit will become rented for each $10 increase in rent. What rent should the manager charge to maximize his monthly income