Algebra problem involving standard notation & %.

Please help me solve these three problems:

The human body can produce 2.5 million blood cells per second. If a person donates 500mL of blood, how long will it take to replace the red blood cells if a litre of blood contains 5 * (10^12) red blood cells?

The Sun is approximately 1.5 * (10^8) km from Earth and a comet is approximately 3 * (10^6) km from Earth. How many times further from Earth than the comet is the Sun?

A music store sells three types of CDs: classical, blues and heavy metal. In one week the sold a total of 420 CDs. They sold 10% more classical than blues whiles sales of heavy metal constituted 50% more than the combined sales of classical and blues CDs. How many of each type of CD did they sell?

Thanks in advance!