Riscrivilo così:
tratta i dati come float e scanf per dare un input

/* Calculating compound interest */
#include <stdio.h>
#include <math.h>

main()
{
int year;
float amount, principal = 1000.0, rate = .05;

scanf("%f",&principal);

for (year = 1; year <= 10; year++) {
amount = principal * pow(1.0 + rate, year);
printf("%4d%21.2f\n", year, amount);
}

return 0;
}