Fibonacci Formula, How to round in C++?

Ok, my question is simple, how do you round in c++? im sure there is some prebuilt function to do this, least i hope.

What im trying to do is build a test for figuring out a simple Fibonacci formula, (n-1+n-2)=x type thing. Basically you can do it perfectly through Recursion, but as all you know far better than me (assuming i got this right), with big numbers you end up having thousands of instances of the function to complete your Fabonacci formula.

What i wanna do is figure the percentage of a number and round it to zero, i wanna see where (61%of n+40% of n)=x starts going wrong. Cuz rounded to zero, in small numbers that works just as good as recursion, im just curious as to what number it will go off on. Thus i need to know how to round in C++.

Thanks :)

(if you dont know what Fabonacci is dont worry bout it, all i need to know is how to round in c++ lol).

Also, sidenote, here is an example of a Fab Formula.

1,1,2,3,5,8,13,21,34,55,89,144,223..