#include <stdio.h>
#include <math.h>
#include <stdlib.h>
int main(void){
	int T,k;
	double now=0,tmp;
	double a,b,x;
	scanf("%d",&T);
	while(T--){
		scanf("%lf %lf %lf",&a,&b,&x);
		if(a<x){
			printf("-1\n");
			continue;
		}
		k=(int)ceil(a/b);
		now=0;
		while(now<x){
			int cnt=2*k-1;
			tmp=(a-b*(k-1))/cnt;
			tmp=(x-now>tmp?tmp:x-now);
			now+=tmp;
			a-=cnt*tmp;
			if(x-now>b&&a<b){
				a=-1;
				break;
			}
			k--;
		}
		if(a<0)printf("-1\n");
		else printf("%.5lf\n",a);
	}
	return 0;
}