NormalizationDate: 08/01/2001 at 13:12:00 From: michael robinson Subject: Algebra How do I figure out this question? 90 + 70 + 88 + 94 + x / 5 = 85 I can get it to 3.42 / 5 = 85 but then I am stuck. Date: 08/01/2001 at 16:22:11 From: Doctor Ian Subject: Re: Algebra Hi Michael, It looks as if you meant to write 90 + 70 + 88 + 94 + x --------------------- = 85 5 In other words, you want to find x such that the average is 85. If you add up all the numbers on the left, you get 342 + x ------- = 85 5 which is sort of like what you got, except it looks as if you forgot about the x. (Oops!) From here, you can 1. multiply both sides of the equation by 5, and then 2. subtract 342 from both sides of the equation to get your answer. But there is an easier way to do this. Note that if the average is 85, then 90 + 70 + 88 + 94 + x 85 + 85 + 85 + 85 + 85 --------------------- = ---------------------- 5 5 Now, if we subtract 85 from each term, we get 5 + (-15) + 3 + 9 + x 0 + 0 + 0 + 0 + 0 --------------------- = ----------------- 5 5 Now we have smaller numbers, which are easier to deal with: 17 - 15 + x ----------- = 0 5 2 + x = 0 which tells you that x = -2... which, in our new context, means that x is two less than 85, or 83. What's going on here? It's called normalization. We could draw the original figures as a bar graph: 85 v 90 ||||||||||||||||||||||||||||||||||||||||||||| 70 ||||||||||||||||||||||||||||||||||| 88 |||||||||||||||||||||||||||||||||||||||||||| 94 ||||||||||||||||||||||||||||||||||||||||||||||| x ? We want to know what value to choose for x so that the average is 85. Now, what if we subtract 20 from everything? 65 v 70 ||||||||||||||||||||||||||||||||||| 50 ||||||||||||||||||||||||| 68 |||||||||||||||||||||||||||||||||| 74 ||||||||||||||||||||||||||||||||||||| x ? We get basically the same picture, but shifted to the right. Do you see why the average here will be 20 less than the average in the original picture? What I did in the 'easy' method was to subtract the average from each term, to get the difference from the average: 5 0||||| -15 |||||||||||||||0 3 0||| 9 0||||||||| x ? Now it's easier to see that the other items add up to slightly more than zero, which means that x is going to have to be slightly less than zero. But when you just use numbers, what it means is that if you add up all the other normalized numbers, you know that x has to be whatever it would take to get that sum to be zero. Which is to say, it's the sum of the other numbers multiplied by -1! Which way is 'easier' depends on which operations you're more comfortable with. I don't mind doing normalizations in my head, but I hate multiplying numbers bigger than about 12, because I tend to make silly mistakes (even when I use a calculator). Other people find normalizations difficult to understand, but they don't mind multiplying big numbers. There's always more than one way to solve a problem, and it's important to learn which methods work best for you. One nice thing about solving a problem in two different ways, though, is that if both methods give you the same result, you can be pretty confident that it's correct. I hope this helps. Write back if you'd like to talk about this some more, or if you have any other questions. - Doctor Ian, The Math Forum http://mathforum.org/dr.math/ |
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