calculating a string equation

[Astan Chee]

Hi,
I have some variables in my script that looks like this:
vars = {'var_a':'10','var_b':'4'}
eqat = "(var_a/2.0) <= var_b"
result = "(var_a+var_b)/7"
What I'm trying to do is to plug in var_a and var_b's values from vars 
into eqat and see if eqat returns true or false as well as getting the 
value of result if these variables were "plugged in". How do I do this?
I'm also expecting eqat and result to contain various python 
mathematical operators like **, and compounded ()'s.
I'm not sure how to convert the equation; if I have to make a bunch of 
if-statements or if there is a python function that already does 
something like this.
Thanks for any help.
Cheers
Astan