Converting Calendar Months Effort to Percent Effort - Can you please help?

[lstrand]

Dear Math wizards,
Your help with the following would be sincerely appreciated.

When converting calendar month effort to percent effort I use the following equation: project period / calendar months (e.g. 12 months / 1.2 Cal Months = 10%)

However, how do I convert 0.90 Cal Months to percent? (12 months / 0.90 = 13.33333.% which is not right. (e.g. 12 months x 13.3% = 1.59 Cal Months or rounded to 1.6 Cal Months.) :-(

Could you please explain, in simple english, how to do this?

Thank-you so much :-(

 

[Otis]

Hi Istrand. It seems like your ratio may be inverted. You have:

12.0/1.2 = 10.0

But 10.0 is not the decimal form of 10%.

10.0 = 1000%

The percent sign means "per hundred", so 1000% means 1000/100 (which is 10).

See: https://www.mathsisfun.com/percentage.html

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Invert the ratio:

1.2/12.0 = 0.10

0.10 is the decimal form of 10% because 10% means 10/100.

12 × 10% = 1.2

Therefore, 1.2 is 10% of 12.

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0.90/12.0 = 0.075

0.075 is the decimal form of 7.5% because 7.5% means 7.5/100

12.0 × 7.5% = 0.90

Therefore, 0.90 is 7.5% of 12.

Does this look like what you're trying to do? That is, are you trying to find what percent of 12 months a specific time interval is?



PS: To write a decimal number (like 0.10 or 0.075) as a percent, we multiply by 100. A mental shortcut for multiplication by 100 is to shift the decimal point two positions to the right.

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