Formula to figure out grade caclulations

[logan_fernandes]

I've been tasked with creating a spreadsheet and I can't figure out this formula. I know it's simple but I am just not seeing.

Essentially, what I'm making is a grade calculator, with only 3 types of grades: 100%, 0%, and -100%.

To figure out the percentage score, I've come up with (100's - 0's) / total grades. This is working perfectly to get the score, however, what I need to make up is a formula to find out how many more 100's it would take to get to a 90%

For example, if we have 21 100's and zero 0's and 25 total scores, then (21-0)/25 = 84%.

From that I just need to figure out the formula to tell me what I would need to get to 90% and I'm horrible at algebra...lol

[shg]

For example, if we have 21 100's and zero 0's and 25 total scores, then (21-0)/25 = 84%.

If you have 21 100s and zero 0s, you have 21 total scores, not 25.

[logan_fernandes]

Your right, I messed up my typing there, i meant 21 100's and 0 -100's, which only leaves the 0's that are not factored in anywhere but the total.

[shg]

If you have A 100s, B -100s, and T total scores (100s + 0s + -100s), then to get an average of 90 you need

=10*(0.9*T - A + B), or 15 for your example.

If you have further questions, you might try http://mathforum.org/dr.math/

[logan_fernandes]

Awesome, that totally worked, you rock.