what is the error in the line of ci=?

Question

#include<stdio.h>
#include<conio.h>
#include<math.h>
int main()
{
    int p,r,t;
    float si,ci;
    printf("Enter principal\n");
    scanf("%d",&p);
    printf("Enter rate\n");
    scanf("%d",&r);
    printf("Enter time\n");
    scanf("%d",&t);
    si=(p*r*t)/100;
    printf("simple interest %f\n",si);
    ci=p*pow((1+r/100),t);
    printf("compound interest %f",ci);
    return 0;
}

Answer 1

r / 100

is an integral division, meaning only the integral (no fractions) part is used in this calculation. E.g. 123/100 = 1 (and 0.23 is throws away).

You probably want to do a floating point division here instead:

r / 100.0

Or r / 100.0f, (notice the 'f' suffix) if you want to explicitly match the type float, otherwise the compiler will transform the double 100.0 into a float automatically.