Friday 16 August 2013

I'm not getting the correct sum for this geometric progression word problem.



This is another question my teacher asked to do as homework. I've been getting the sum for this word problem.

Here's the word problem:

A man on a diet loses 1.5% of his weight during each week.
(a) If he initially weighs 150kg, write down his body weight at the end of each of the first 5 weeks.
Here's my working out:

My working ou for (a)

(b) How much does he lose in total during that time?
This is the part of the question I've been trying and have been getting the wrong sum. Here's my working out:
Working out for (b)

The sum for alternate (b) is approximately 727.84kg and it clearly doesn't make sense. I checked the back of the text book where my teacher got this question from and the answer is 10kg. I did the question over and over again and kept coming up with 727.84kg. How do I get 10kg?


Answer



Just in case you are wondering, you can get 139.0824754 by doing:
$$150\times(1-\frac{1.5}{100})^5$$
The 1- bit gets you 0.985, and you can use a this method, a similar method to compound interest, to find the answer of 139.08..... then to get 10.9175
Hope this helped!


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