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;;