I often wake myself up in the morning by giving myself mental math problems while in the shower. Math is generally difficult for me, so this is a way for me to get my brain started in the morning.
I tried this one today. If the item I want to buy is marked $100, and I know it was marked down 20%, what was the original price?
Now, I know that if I want to find out what 20% of 100 is, I can do that in a number of ways (20% is easy, but other percentages are not, so I know the formula).
I can’t seem to get what the original price would be, though. Through trial and error, I can see that it would be $125 (20% of 125 is 25, 125-25=100) but I know there is a mathematical way to figure this out without trial and error.
Any help math dopers?
Algebra is your friend. Start with the known, $100 is 80% or 8/10 of the original price.
100 = x * 8 / 10
multiply both sides by 10
1000 = x * 8
divide both sides by 8
125 = x
I knew there was something wrong with focusing on 20%, but I couldn’t quite get it straight.
Thanks… I’m going to assume you weren’t being snarky.
What TheLadyLion said. Or, to spell it out in even more detail:
Your question is: What number, minus 20% of that number, is $100?
Let x stand for the number. Then your question translates into
x – .20x = 100
Combine the x terms: 1x - 0.20x = (1 – 0.20)x = 0.80x, so
.80x = 100
Then divide both sides by .80 to get
x = 100 / .80 = 125
Heck, I couldn’t help but “plug” my own site on this one.
Go here: http://www.1728.com/percent2.htm
scroll down to Percent Decrease Calculator and it is an Example ‘C’ type problem.
Needless to say, I came up with $125 also.