HELLO: Wrong answer

I am getting a wrong answer for HELLO. The logic I used was that instead of calculating for one month, I calculated it for 36 months(as that was the max validity option)and then compare the prices. The sample cases given work fine. But I guess I am missing some corner case. Please help. Thank you!

My code ::

#include <iostream>
#include <stdio.h>
using namespace std;
int main()
{
    int cases;
    float default_rate, callRate;
    int minsLastMonth;
    int addons;
    int validity, planCost;
    float cost;
    scanf("%d", &cases);
    while(cases--)
    {
        scanf("%f %d %d", &default_rate, &minsLastMonth, &addons);
        float expenditure = minsLastMonth*default_rate*36;
        float maxProfit=0;
        int index=0;
        for(int i=0; i<addons; i++)
        {
            scanf("%d %f %d", &validity, &callRate, &planCost);
            cost=((36/validity)*planCost)+(callRate*minsLastMonth*36);
            if(cost<expenditure && (expenditure-cost)>maxProfit)
                {
                    index=i+1;
                    maxProfit=expenditure-cost;
                }
        }
        printf("%d\n", index);
    }
    return 0;
}