Something

aport@hal9000:~$ cat test.c
#include <stdio.h>

/* print fahrenheit-celcius table */

double convert(int a)
{
	double u;

	u = (a - 32) * (5.0/9.0);
	return u;
}

int main()
{
	int f;

	for (f = 0; f <= 300; f = f + 20)
		printf("%3d %6.1f\n", f, convert(f));

	return 0;
}
aport@hal9000:~$ gcc -Wall -Werror -pedantic -std=gnu99 -o test test.c && ./test
  0  -17.8
 20   -6.7
 40    4.4
 60   15.6
 80   26.7
100   37.8
120   48.9
140   60.0
160   71.1
180   82.2
200   93.3
220  104.4
240  115.6
260  126.7
280  137.8
300  148.9