it is already there, here is how you do it:
HUNGER is the variable name, lets the value of hunger is 20, and you want that as a percent, on the pages description, type:
% %%HUNGER%%, now this is confusing but the first "%" is the "% 20", the "%%HUNGER%% is how you display the variable, so if you were to just say %%HUNGER%%, it would display 20, but because the extra % is there, it displays % 20. Now if you wanted to have, if hunger is below 40, you should eat something soon, this is it:
%%HUNGER%<%40%You Should eat something soon%%
Note that hunger is the variable and it appears first, the operator, <, means less than, and its less than 40, the text afterwards is what is displayed.