6.7

#include<stdio.h>
#define  CURRENT 100
int main()
{
    int ni;
    double rate;/*工场产值的增长率*/
    double op;/*工场总产值*/
    printf("Input rate:");
    scanf("%lf",&rate);
    op=CURRENT;
    for(ni=0;op<2*CURRENT;ni++)
    {
        op=op*(1+rate);
    }
    printf("when rate is%.0f%%,the op can be doubled after %d ni.\n",rate*100,ni);
    return 0;
}