A swimming pool holds 10,000 gallons of water. It can be filled at the rate of 100 gal/min and emptied at the same rate. Right now, the pool is filled, but there are 20 pounds of an impurity dissolved in the water. For the safety of the swimmers, this must be reduced to less than 1 pound.
a) Assuming pure water is being pumped in, how long until the pool can be used by swimmers again. 
b) If it takes 200 minutes to empty the pool and completely refill it. Should the life guards drain and refill the pool OR slowly purify the water as in a) to allow swimmers to get back in soonest?