# Thread: I need help with a calculation thats baffling me! (easily done!)

1. ## I need help with a calculation thats baffling me! (easily done!)

Hi all,

i'm not a mathematician therefore I need help! is there a calculation that determines how many times a smaller number has to be doubled to achieve a larger number? also will it work where doubling a number doesn't exactly fit into the larger number?

for example I know that 10 doubled three times equals 80 but is there a calculation where we know of 10 & 80 but it calculates that 10 needs to be calculated 3 times?

Cheers for any help!

Morgan

2. Use logarithms.
If you need to find x in:
$2^x = y$
then $x = \log_2(y)$

Note: On a calculator use log(y)/log(2) to get $\log_2(y)$

The questions you are asking is $10\times 2^n = 80$.
Simplify this into $2^n = 8$.
Then $n = \log_2(8) = 3$.

3. Thanks, sounds complicated!!

How would I type this into a spreadsheet to give the answer in the box?

Sorry to be a pain!

4. If you use something like Excel, then they have the logarithm function.
I think it is in Insert -> Function -> Math and Trig functions -> log

Then if you want to see how many times A needs to be doubled for B type

= log(B/A)/log(2)

where "/" is division
and "log(...)" is any log function in Excel.

It may give you decimals if it is not exactly double.

5. Wow!

Awesome stuff, big thanks for your help!!!