Calculate number based on percentage

D

dan.gates

Hello -
My algebra days seem to be a little hazy today. Could someone please
assist on the basic calculation to find the original number.

example 1:
This years sales are $3700, a decrease of 11.6%. What would last years
sales be?

example 2:
This years sales are $4500, an increase of 151%. What would last years
sales be?

Thanks for any insight on this calculation.
Dan
 
M

Manish Bajpai

Dan, answers to your examples are as under:

1. This years sales are $3700, a decrease of 11.6%. Last years
sales would be $4185.52.

Formula : =3700/(100%-11.60%)

2.This years sales are $4500, an increase of 151%. Last years
sales would be $2980.13

Formula : 4500/151%

Thanks,

Manish Bajpai
 
D

David Billigmeier

The answer for #2 is wrong, you forgot to add a 1 to the 151%. The formula is:
4500/2.51
 
D

Dan

Thanks, this is great!

Formula : =3700/(100%-11.60%)
Formula : 4500/151%

Is there one general formula to use for both scenerios, so I wouldn't
have to differentiate between positive and negative changes?
 
D

David Billigmeier

Ya, in your case you can use the formula:

<Current Year Sales> = <Last Years Sales> * (1+<percentage change>)

With variables located within <>. Just solve for any variable you are
trying to find given you know the other 2. Note that if you have a decrease
in percentage you use a negative number for <percentage change>
 
M

Manish Bajpai

David,

Dan's response proves that I was right in dividing 4500/151%. Dan was
looking for 151% increase and not 251% increase.

Dan,

It is my pleasure.

Why do you worry for future, when we guys are here to help you out.

Manish Bajpai
 
D

David Billigmeier

Manish:
151% increase DOES NOT mean multiplying a number by 1.51, it means
multiplying by 2.51. Check it for yourself:

Take a simple example: 150% increase from 2 means doubling 2 one time and
then adding another 50%... obtaining 2+2+1=5. My answer would provide the
right answer, or 2*(150%+1)=2*(1.5+1)=2*2.5=5. Your answer would only yield
3.

Look at the formula you gave for #1, 3700/(100%-11.60%). You have to do the
same thing for increases... you only had 4500/151%, you need
4500/(100%+151%)=4500/(2.51)
 
D

Dan

Hmm, this "general" formula doesn't seem to work for the negative
changes. Any other thoughts?
 
D

David Billigmeier

Dan -
It actually will work, just use like so:

<Current Year Sales> = <Last Years Sales> * (1+<percentage change>)

So in your example, you had current year sales of 3700 and a decrease of
11.6%, so the formula would become (remember I said use a negative percentage
for decreases):
3700 = <Last Years Sales> * (1+(-.116))
3700 = <Last Years Sales> * (1-.116)
3700 = <Last Years Sales> * .884
Last Years Sales = 3700/.884
Last Years Sales = 4185.52

If you are getting confused by Manish (when he says an increase in sales by
151% means multiplying by 1.51), he is wrong... look:

Once again use your information:

4500 = <Last Years Sales> * (1 + 1.51)
4500 = <Last Years Sales> * 2.51
Last Years Sales = 4500/2.51
Last Years Sales = 1792.83.

Hope this helps,
 
M

Manish Bajpai

Dan,

No point in memoraizing a formula.

Simply understand the logic and you will never miss it:

If last year's sales = X,

Rate of increase or decrease = r%, and

This Year's revenue = 1000.

Then, 1000=X*(100%+r%) ===> note if r is to be mentioned with its symbol
i.e.ngeative for a decrease and positive for increase.

Therefore,

X = 1000/(100% + r%)

Please make sure while arrivng at r% that whether your this year's revenue
is grown by 51% over last year or grown by 151% over last year. If it ilater
case i.e. grown by 151% over last year then David is right.

Thanks,

Manish Bajpai
 
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