Percentage question

[joebell]

This should be easy but for some reason I just can't think of the formula. Here is a simple example ... If I know that I want to have $1,000.00 left over after I give away 10% of my money, how much do I need to start with? Thanks, Joe

[MarvinP]

Hi Joebell,

I used to teach math. The first step to this problem is to guess an answer. I'd guess something like $1500 because I know it needs to be 1000 when I get done. Then 10% of 1500 = 150. So I start with 1500 and give away 150 or I have left 1500 - 150 = 1350. So Darn!! I need it to be exactly 1000 not 1350.

Next guess again and try something less like 1100 and 10% is 110 so is 1100-110 = 1000?? Nope again. So WTF do I start with????

Here is the trick... Instead of starting with 1500 or 1100 you start with X dollars and then you subtract 10% . This looks like X - .1*X is supposed to equal 1000. If you know algebra you can factor out that X from the left and X - .1X is the same as (1-.1)X = 1000. convert the (1-.1) to .9... So .9X = 1000... Divide both sides by .9 and X = 1,111.111 repeat....

Did all that make sense??? The first part of most problems is to guess an answer. Do it lots of times so you can see what you do with the number to see if you guessed correctly. When you can't guess the correct answer, substitute an X for your guess and solve.

Hope that helps.