Math question - weighted average 1

[sabrepoint]

Hi All

Not really electrical engineering, but I'm trying to apply formula to engineering problem ...

The following formula determines the Weighted Average Life (Lavg) of a group of power poles ...

Lavg = Qconc/Qtot * 60 + (Qtot-Qconc)/Qtot * 45

where Qconc is quantity of concrete poles, Qtot is the total of all poles (ie: concrete and wooden), 60 is the standard life of concrete poles and 45 the standard life of the remaining (wood) poles. All OK so far if we know the pole quantities.

My problem is going backwards ie: I know Lavg and Qtot but need to determine Qconc. So the question is, how to rearrange to formula to give Qconc?

Your help is most appreciated.
Graeme

 

[2dye4]

Qconc=Lavg/(60/Qtot-45/Qtot)+45)

Margin of error %90

 

[desertfox]

Hi sabrepoint

I took the numbers 60 and 45 to be multiplying the whole of their respective adjacent ratio's and therefore get an expression:-


((Lavg - 45)* Qtot)/15 = Qconc



regards desertfox

 

[sabrepoint]

You're right, thanks desertfox!