Price of Pencils
Date: 10/07/2002 at 10:06:13 From: George Subject: Word problems The price of pencils has skyrocketed in recent years. Each year for the last 7 years the price has increased, and the new price is the sum of the prices for the two previous years. Last year a pencil cost 60 cents. How much does a pencil cost today? How much did a pencil cost 7 years ago? I am trying to help my daughter on this math problem but am having no luck. Can you help? My daughter is 11 years old and in 6th grade. Regards, George
Date: 10/10/2002 at 10:26:15 From: Doctor Peterson Subject: Re: Word problems Hi, George. This is an interesting problem! At first I thought there couldn't be enough information, but then I realized that the fact that the price increased every year was the key. The problem involves a Fibonacci sequence, working backward; the most orderly way I can see to do it is to use simple algebra. The main idea is that, since in each year the price is the sum of the prices of the two previous years, the increase in price is always the price the year before. For example, if the price started at 5, and the previous year it was 2, then the next year it will be 5+2=7, and the following year it will be 7+5=12. Working backward, each year's price is the difference between the next two years: 12-7=5. Let's work backward, starting with the unknown price this year, which we'll call x. Last year it was 60, so the previous year it must have been x-60 (so that the sum of x-60 and 60 gives the next price, x). Continuing this process backwards gives us a sequence of expressions: x 60 x-60 120-x 2x-180 300-3x 5x-480 780-8x 13x-1260 I've continued this to the eighth year back, since that price is the increase from the seventh year. Now we use the key: all of these increases must be positive. That gives us a sequence of inequalities, each of which can be solved to find a range for x: x > 0 60 > 0 x-60 > 0 x > 60 120-x > 0 x < 120 2x-180 > 0 x > 90 300-3x > 0 x < 100 5x-480 > 0 x > 96 780-8x > 0 x < 97.5 13x-1260 > 0 x > 96.92 Notice that alternate inequalities give lower and upper bounds on what x can be, getting tighter and tighter; by the end, we see that 96.92 < x < 97.5 and the only whole number value x can have is 97. Now, plugging this into the expressions for the prices (or just working backward numerically by subtracting, which is easier), we get x = 97 60 = 60 x-60 = 37 120-x = 23 2x-180 = 14 300-3x = 9 5x-480 = 5 780-8x = 4 13x-1260 = 1 Seven years ago, the price was 4 cents, and it has increased every year since, following the indicated pattern. Of course, if you just have an answer, you can check it by working forward and seeing if the results are correct. And I suspect that the method an 11-year-old is expected to use is more like trial and error: try setting the original price at 1, 2, 3, 4 and trying various small increments for the next year until you find one that works. Basically, this is just a long addition problem. If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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