If the DVDs were the same amount of money and he bought 4 of them and your total is 79.94 dollars you have to divide four from it because the DVDs are the same cost then your going to subtract 5.98 because that is shipping and your trying to find the total of the DVD and you get 14 dollars and 5 cents.
Based in your question where as the oil is pumped continuously from the well at a rate proportional to the amount of the oil left in the well. so the answer of the following questions are: A.1 is to 200,000 B.5 years
