Develop an estimate for each of the following situations.
(a) The cost of a 500- mile automobile trip, if gasoline is $4.00 per gallon, vehicle wear and tear is $0.65 per mile, and our vehicle gets 25 miles per gallon.
(b) The total number of hours in the average human life, if the average life is 75 years.
(c) The number of days it takes to travel around the equator using a hot air balloon, if the balloon averages 100 miles per day, the diameter of the earth is ~4,000 miles.
(d) The total area in square miles of the United States of America, if Kansas is an average-sized state. Kansas has an area of 390 miles *200 miles.