[1] 4 x 5 = 20
y = 3
i = 5
x = 2.5
y = 8
x = 3.0

[2] 5 x 4 = 20
a)
1)
ans1 = 90
ans1 = 100
ans2 = 110
2)
ans1 = 24
ans1 = 34
ans1 = 44
ans2 = 44

b)
1)
ans3 = 90
ans3 = 100
ans4 = 110
2)
ans3 = 24
ans3 = 34
ans3 = 44
ans3 = 54
ans4 = 54

[3] 10
n = 6

[4] 10x2 = 20
a)
x-x-x-x*x

b)
switch(a){
    case ' ':
    case '+':
        a = '-';
        break;
    case '/':
        a = '*';
        break;
    default:
        a = 'x';
}

[5] 15

#include <stdio.h>
int sub(int n);
main()
{
     int num=10;
     printf("NUM = %d\n", sub(num));
}
int sub(int n)
{
     return n*n*n;
}

[6] 15

p = a;
q = b;
while(*p != '\0')
   *q++ = *p++;
*q = *p;

あるいは、

p = a;
q = b;
do{
   *q++ = *p;
while(*p++ != '\0')