C语言: 3. 输入20个数,计算这20个数的平均值并输出,然后按每行4个数据...答:int main() { int i;double n[20];double sum=0.0,average;for (i=0;i<20;i+=4) { printf("Input %d - %d number[n1,n2,n3,n4]:", i+1, i+4);scanf("%lf,%lf,%lf,%lf", &n[i],&n[i+1],&n[i+2],&n[i+3]);} for (i=0;i<20;i++) { sum += n[i...