Try this:
int main(int argc, char *argv[])
{
float c,f;
printf("plesase write a number of degrees celsius:");
if (scanf("%f",&c) != 1)
{
printf("Error\n");
}
else
{
f=c*9/5+32;
printf("number of degree fahrenheit: %f\n",f);
}
system("PAUSE");
return 0;
}