Alfred Ratio Calculator
Easily calculate the missing number in a ratio. Given the equation 2/4=?/10, type rr 2 4 x 10 and get 5!
Installation

Doubleclick on the Ratio Calculator.alfredworkflow file you just downloaded to add it to Alfred.

Click the Import button.

The default trigger keyword is
rr
. Change this as you see fit. To do so, doubleclick the leftmost of the two blocks that represent the workflowthat is, the one that says 'Script Filter'and change the 'Keyword' field to your preferred trigger.
Usage

Launch Alfred.

Start the ratio calculator by typing
rr
and a space. 
Type a ratio equation as a series of three numbers and one nonnumber to indicate the missing value.
For example, to find the missing number in the equation 2/4=x/10, type:
2 4 x 10
You can use
x
,?
, or any nonnumber character to indicate the number you want to find. 
The result will show up in the feedback list below as you finish your equation. Hit Return to copy the result to your clipboard.
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See Also
This Alfred workflow is based on my ratio calculator script for the command line, which functions identically but includes several options.
Thanks
Many thanks to Cody Robbins for code review, refactoring, and other helpful contributions.