Please have a look at my code for help, and then attempt it yourself. Otherwise he might be suspicious of cheating and/or you'll never learn C. I'd normally flame, but I'm in a good mood [tired].
Code:
#include <stdio.h>
#define FRANKS 5.05
/* this means anytime you type FRANKS, it is replaced with 5.05 */
#define DM 1.47
/* same here except with DM */
int main(void)
{
float input;
int choice;
printf("Enter amount in dollars $");
scanf("%f",&input);
/* get the floating point (%f) input, this is a number with a
decimal point (although it doesnt need one) */
printf("Convert to (1)Franks or (2)Deutsche Marks? >");
scanf("%d",&choice);
/* get a decimal integer (%d) input, no decimal point */
if(choice == 1)
printf("$%.2f = %.2fF",input,input*FRANKS);
/* * is the symbol for multiplication */
else if(choice == 2)
printf("$%.2f = %.2fDM",input,input*DM);
else
printf("Invalid choice.");
return 0;
}