C :

#include<stdio.h>
#define CURRENT 100
main()
{
	int year;
	double growRate;
	double output;
	scanf("%lf",&growRate);
	output = CURRENT;
	for(year = 0;output < 2*CURRENT;year++)
	{
		output = output * (1+growRate);
	}
	printf("%d",year);
	return 0;
} 

C++ :

#include<stdio.h>
#define CURRENT 100
main()
{
	int year;
	double growRate;
	double output;
	scanf("%lf",&growRate);
	output = CURRENT;
	for(year = 0;output < 2*CURRENT;year++)
	{
		output = output * (1+growRate);
	}
	printf("%d",year);
	return 0;
}