How do you calculate interest on a loan with different rates?

I think it's obvious I don't understand. How do you tell the computer in C to decide which is the appropriate interest rate and then calculate and display it. This is the best I could come up with and I have to hand this as an assignment tomorrow. I had not clue it would be this difficult.

#include <stdio.h>
int main (void)
{
    float time;
    float principal;
    char response[15];
    float rate1, rate2,rate3,rate4;
    rate1=.04,rate2=.05,rate3=.06,rate4=.07;

    float SimpleInterest1;
    float SimpleInterest2;
    float SimpleInterest3;
    float SimpleInterest4;

    SimpleInterest1=principal*time*rate1;
    SimpleInterest2=principal*time*rate2;
    SimpleInterest1=principal*time*rate3;
    SimpleInterest2=principal*time*rate4;


    printf("Please enter principal\n");
    scanf ("%f",&principal);
    printf ("Please enter time\n");
    scanf ("%f",&time);

    if (principal <= 5000)
{

    printf ("%f",&SimpleInterest1);
}
    printf ("Do you still want the loan?\n");
    scanf ("%s",response);

    return 0;
}