Rate of Change from June and September

Question

From April through December 2012, the stock price of QRS Company had a roller coaster ride. The chart below indicates the price of the stock at the beginning of each month during that period. Find the monthly average rate of change in price between June and September.
Month Price
April (x=1) 116
May 108
June 87
July 100
August 94
September 111
October 92
November 84
December 65

Answer 1

The monthly average rate of change is given by the change in the y-value (which is price in this case) divided by the change in the x-value (which is month number in this case).  This is written as  

 

(y₂ - y₁) / ( x₂ - x₁)  

 

y1 = 87    (price in June)

y₂ = 111  (price in September)

 

x₁ = 3  (since x = 1 in April and June is 2 months after April)  

x₂ = 6  (since September is 5 months after April)  

 

So the average rate of change is (111 - 87) / (6 - 3) = 24 / 3 = 8