Need some help with percentages

Hi all,

I'm really struggling with a percentage problem for a new spreadsheet I'm developing to support me achieving my fitness and weight goals. I'm hoping to develop something which will help others to lose weight as I think I have a really good system for losing and maintaining weight. Here goes:

I'm trying to calculate how many weeks it will take me to achieve my **body fat** goals following the nutrition programme I've just designed. The measure I'm struggling with is called '**Lean Body Mass**' which is essentially your **weight** minus you **Body Fat Mass**; here's an example:

Weight in kg: 71kg

Body Fat Percentage (BFP): 12%

Body Fat Mass = Weight – (Weight*BFP), or in my example Body Fat Mass = 71-(71*0.12) = 8.5kg, therefore,

Lean Body Mass = Weight – Body Fat Mass, i.e., Lean Body Mass = 71-8.5 = 62.5kg

The complexity comes in when I try to calculate what my target **Body Fat Mass** is should my BFP be 7%. It sounds easy because we know what my Lean Body Mass is, but you can't just add 7% to BFP as it gives an incorrect figure. For example, if we try to reverse engineer the first problem it works out like this:

Weight = Lean Body Mass * BFP, or in our example, Weight = 62.5*1.12 = 70kg. The unknown factor is the __total new weight__ so this causes the problem.

In summary, I need to know how to calculate my 71kg starting weight from my Lean Body Mass of 62.5kg and my known BFP of 12%.

Can someone please help as this is driving me crazy?

Thanks, Lee