ternary operator(?), vectors, and recursive function

I came across this little code which uses the ternary operator, vectors and a recursive function. I think I can see how the ternary operator ("?:") is used, but I dont understand several things:
1. What controls when the doit function is exited?
2. In the doit function which member of the p vector is called
with "/100,p)" ?
Can anyone help explain how this program is working?

Code:

#include <string>
#include <vector>
#include <iostream>
using namespace std;

class Multiplier {
	public:
//
    // a>0: this is amount spent on goods
    // a<0: this the amount spent on services
    int doit(int a, vector <int> &p){        //doit is recursive function ?
        return 
            a>0?  a + doit(a*p[0]/100,p) + doit(-a*p[1]/100,p):
            a<0? -a + doit(-a*p[2]/100,p) + doit(a*p[3]/100,p):0;
        }
        int spending(int amount, vector <int> percent){
            return doit(amount, percent)-amount;
        }
    };
//
    int main(void){
        int amount;
        vector <int> percent;
/*
//0)
23
{20,50,10,40}
//
//
        amount=23;
        percent.push_back(20);
        percent.push_back(50);
        percent.push_back(10);
        percent.push_back(40);
/*
// 1)
100000
{35,60,70,25}
*/
//
        amount=100000;
        percent.push_back(35);
        percent.push_back(60);
        percent.push_back(70);
        percent.push_back(25);
/*
// 2)
5
{19,19,19,19}
//
//
        amount=5;
        percent.push_back(19);
        percent.push_back(19);
        percent.push_back(19);
        percent.push_back(19);
*/
//
         Multiplier p;
         cout<<"Total Add. Expenditure Gen.="<< p.spending(amount,percent);
         cin >> "";     //pause console
         percent.~vector<int>();        //Vector Destructor-frees memory
         return (0);
}

It returns when a==0, this will never happen if one of the first four elements is greater than 100.

It explicitly calls the destructor, this is a big no-no (unless you know exactly what you are doing and even then its mostly a desperate hack) The destructor will be called when percent goes out of scope.

As to why it is doing this, or the correctness of the answer I cannot even begin to fathom. It looks like a quick one-off or homework for a buisness class to me.

Thanks, grib, but what about

2. In the doit function which member of the p vector is called
with "/100,p)" ?