3) Every year Sully buys 130 red pens for grading at the beginning of the year. He figures that the number of pens decreases exponentially at about 9% each week. When he only has 10 left he goes and buys more.
a) Set up an exponential equation for the given situation.
b) Rewrite the equation as a logarithm.
c) Expand your logarithm using Properties of Logarithms.
d) How many weeks until Mr. Sullivan needs to buy more pens?