This seems like such a basic question yet I can't really find the answer 
anywhere in the docs (I'm probably looking in the wrong place).

Anyway I need to compute a percentage and output it. The percentage 
should be in the form 39.45% (i.e. round to the nearest two decimal 
places). So I have

top = 68
bottom = 271

percentage = (top.to_f / bottom * 100 * 100).round.to_f / 100

The above works but it does not seem very intuitive. What I would rather is:

percentage = (top.to_f / bottom).round(4) * 100

But round does not take any arguments on how many decimal places I want. 
It just assumes I don't want any. Perhaps there is another function that 
I am not seeing that will round while keeping a certain number of 
decimal places. Obviously I could also enhance round to take an optional 
argument but I wanted to see if there was an already existing function 
in the Ruby std library that will do it for me.

Thanks,

Eric