Simple math quesiton

Discussion in 'Community Discussion' started by cantthinkofone, Jul 1, 2011.

1. cantthinkofone macrumors 65816

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Jul 25, 2004
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Missouri, USA
#1
I am having a argument with my online,first mistake, math class instructor about a question on a test.

Question is: Guy gets a 20% raise and now makes \$67,200, what was is pervious salary. I said the answer is \$53,760. The test said no it is \$56,000.

I used the method of 67,200 times .2 and got 13,440. Subtracted that from 67,200 and got 53,760. Or another way I could have done it was 10% of 67,200 is 6,720, times two to make it 20% and you get 13,440. 13,440 subtracted from 67,200 is 53,760.

The method she explained I should have used and they have been using is as fallows.

new salary = old salary + raise

67,200 = x + 20% (x) The 20% has to be times the old salary.

67,200 = x + .2 x Change percent to decimal

67,200 = 1.2 x combine like terms

67,200/1.2 = 1.2 x/1.2 Divide by 1.2 to isolate x

\$56,000 = x

Check: 56000 + 56000(20%) = 67200

56000 + 11200 = 67200

So either way you get 67,200 as the new amount. Which is correct? Makes me mad that this question is so simple but is such a cluster.

2. siurpeeman macrumors 603

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#2
\$56,000. it's a raise of 20% of the original amount, not a raise of the new amount.

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4. mobilehaathi macrumors G3

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5. Mac'nCheese macrumors 68040

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Feb 9, 2010
#5
This part of your post is your problem. You are not solving for the new amount. You are solving for the old one. You can do lots of things to the new amount and then check your math and say "well, I got 67,200 so I'm correct." You can subtract \$1000 and then say "66,200 + 1000=67200, so I'm correct."

6. codymac macrumors 6502

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Jun 12, 2009
#6
56k

Put another way...
56000 x 1.2 = 67200
53760 x 1.2 = 64512

Don't argue with her.

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9. -aggie- macrumors P6

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10. teleromeo macrumors 65816

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11. Moof1904 macrumors 65816

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May 20, 2004
#11
Frame of reference

This illustrates one of the easiest traps to fall into with math, no matter the level at which you're studying: keep your frame of reference straight. If a person gets a 20% raise and then ends up at a given salary, the 20% raise will be a raise to the (unknown) starting point.

In contrast, the method of subtracting 20% from the ending salary would correct if the question were 'what results from a 20% pay cut?' because that question uses the final salary as the frame of reference for the 20%.

12. Gregg2 macrumors 603

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May 22, 2008
Location:
Milwaukee, WI
#12
You took 20% of the new salary and then subtracted that from the new salary. Since you took 20% of the higher amount, your answer is less than what the old salary was, of course. It wouldn't be much of a math problem if there was no unknown. You did your calculations with what was known. No self-respecting math instructor is going to ask "What's 2 + 2 ?"

13. yg17 macrumors G5

Joined:
Aug 1, 2004
Location:
St. Louis, MO
#13
Old salary is x. The raise is a 20% increase of his original salary, so the equation is x + .2x = 67000, then solve for x.

x + .2x = 67000
1.2x = 67000
x = (67000/1.2)
x = 56000

14. MrSmith macrumors 68040

Joined:
Nov 27, 2003
#14
I can also confirm 67,200/1.2 = 56,000 according to my calculator.