How to calculate amount to come up with percentage

Can someone help me with a formula for this situation.
A=1
B=0
C=-1
example:
if i have 5a, 1b and 2c..what will be the formula for the average.

and another formula for:

D= #'s of A for the average to reach 80%
example:
*using manual computation*
5a, 1b and 2c, the average is 37.5%, so how many A do i need to reach 80%?

I'm just new with Excel, please help.

Thanks in advance!

kamusta pengshue???

Pengshue, the title should represent something you might enter into a search engine. Something like "Calculating Avg with multiple duplicates" or "How to calculate amount to come up with percentage"

pengshue, welcome to the forum

"Formula Problem" is hardly any more descriptive of what you want, than "help with making a formula" is

Think google search terms. Perhaps "how to calc averages from multiple values?"

edit: ChemistB beat me to it

kamusta pengshue?

eto try this formula:

=IFERROR(COUNTA(A:A)*(80%-AVERAGE(A:A))/(1-80%),0)

I included the IFERROR statement in case you're already at 80%

*walang anuman pengshue!!! (*you're welcome)

if you're satisfied with the resolution please take time to mark this thread as solved & adding a rep to those who've helped you

ingat pengshue

could you attached a sample file pengshue so i can check..

in what range are your values entered?

remember this formula:=IFERROR(COUNTA(A:A)*(80%-AVERAGE(A:A))/(1-80%),0)

assumes that the values are in column A.

I'm not sure how you come up with 37.5% with 5,1,2

With the numbers you want to apply in in B1:B3 and your values in A1:A3

=SUMPRODUCT(A1:A3,B1:B3)/SUM(B1:B3)

= 0.78 or 78%?

attached is a sample file pengshue.

with your example you will need 17 more a's to meet your target of 80%

the formula in cell E1 will tell you how many a's you need

=IFERROR(COUNTA($B:$B)*(80%-AVERAGE($B:$B))/(1-80%),0)

let me know if it works for you.

Ahh, c = -1 not 1. Still now I get Average = 0.333 not 0.375. Close but no cigar

I get 17 for how many additional 1's do you require. Fixed my previous errors.
With original average in B4, required average in B6 (0.8) and number of previous values in B1:B3

=(SUM($B$1:$B$3)*(B6-B4)/(1-B6))

See attached spreadsheet

weird ChemistB..

that should be 1,1,1,1,1,0,-1,-1

and i get 37.5%

I just read the first post wrong. :/ Now everything matches up with your calculations although I set it up differently

Glad you got it working

thanks for the rep ChemistB