Formula help

  • Thread starter Thread starter Randall Roberts
  • Start date Start date
R

Randall Roberts

ok here is what i am trying to do
hopefully someone can help me


A B C
100 200


in a i have the number of succesfull atempts
in b is the total number of attempts
in c i want to display how many more succesfull atempts are needed to
achieve a certain percentage

i know i can use something like
=A1-(B1*0.65) which would give me 30

but this is not accurate since if i add 30 to column a i must also add
30 to column b (30 more succesful attempts also adds 30 more total
attempts)

can anyone give me any sugestions to solve this problem
 
Randall,
The formula you need to work from is:

(A1+x) / (B1+x) = %

With a little algebra you will get:

x=((%*B1)-A1)/(1-%)

You could put you desired percent in column D (as a decimal) and use the formula:

=((D1*B1)-A1)/(1-D1)

or just replace D1 with your percent.

Good Luck,
Mark Graesser
(e-mail address removed)


----- Randall Roberts wrote: -----

ok here is what i am trying to do
hopefully someone can help me


A B C
100 200


in a i have the number of succesfull atempts
in b is the total number of attempts
in c i want to display how many more succesfull atempts are needed to
achieve a certain percentage

i know i can use something like
=A1-(B1*0.65) which would give me 30

but this is not accurate since if i add 30 to column a i must also add
30 to column b (30 more succesful attempts also adds 30 more total
attempts)

can anyone give me any sugestions to solve this problem
 
Randall Roberts said:
ok here is what i am trying to do
hopefully someone can help me


A B C
100 200


in a i have the number of succesfull atempts
in b is the total number of attempts
in c i want to display how many more succesfull atempts are needed to
achieve a certain percentage

i know i can use something like
=A1-(B1*0.65) which would give me 30

but this is not accurate since if i add 30 to column a i must also add
30 to column b (30 more succesful attempts also adds 30 more total
attempts)

can anyone give me any sugestions to solve this problem
Click to expand...

You have started by recording the wrong data. If you recorded successful
attempts in A1 and unsuccessful attempts in B1, you would only have to
increase one or the other, not both, for each extra attempt. Total attempts
can be calculated as necessary in the formula.

If you also put 0.65 in C1 (the required success rate), you can calculate
the required answer (how many more successful attempts are needed to achieve
this success rate) with this formula:
=(C1*(A1+B1)-A1)/(1-C1)
which would give 85.714.....
or better
=ROUNDUP((C1*(A1+B1)-A1)/(1-C1),0)
which would give 86.

Clearly this is correct, as you would then have 186 successful and 100
unsuccessful attempts, making 286 in total. 186 out of 286 is approximately
65.035%.
 
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