It cannot determine the parameter form of the back itself.

20/7 is an integer (20/7) according to the operation form; There is %d, and then an integer is output.

If you do this:

printf("%f\n "，20/7)；

20/7 is an integer (20/7) according to the operation form; There is %f, after which it is output as a real type; There will be problems, but you can solve them like this.

printf("%f\n "，(double)20/7)；

Or add .0 to make it real.

Note: void main () is best written as int main(void) followed by return 0；;