Determining the big-O runtimes of these different loops?

I have a series of questions in which I need feedback and answers. I will comment as to what I think, this is not a homework assignment but rather preparation for my exam.

My main problem is determining the iterations of a loop for different cases. How would go about attempting to figure that out?

Evaluate Running time.

Q2.

 for(int i =0 ; i < =n ; i++) // runs n times
   for(int j =1; j<= i * i; j++) // same reasoning as 1. n^2
      if (j % i == 0)
         for(int k = 0; k<j; k++) // runs n^2 times? <- same reasoning as above.
            sum++;

Correct Answer: N × N2 × N = O(N^4)

For the following Questions below, I do not have the correct answers.

Q3. a)

     int x=0; //constant
     for(int i=4*n; i>=1; i--) //runs n times, disregard the constant
         x=x+2*i;

My Answer: O(n)

b) Assume for simplicity that n = 3^k

    int z=0;
    int x=0;
    for (int i=1; i<=n; i=i*3){ // runs n/3 times? how does it effect final answer?
       z = z+5;
       z++;
       x = 2*x;
    }

My Answer: O(n)

c) Assume for simplicity that n = k^2,

   int y=0; 
   for(int j=1; j*j<=n; j++) //runs O(logn)?  j <= (n)^1/2
   y++; //constant

My Answer: O(logn)

d)

  int b=0; //constant
  for(int i=n; i>0; i--) //n times
    for(int j=0; j<i; j++) // runs n+ n-1 +...+ 1. O(n^2) 
      b=b+5;

My Answer: O(n^3)

(e)

 int y=1;
 int j=0;
 for(j=1; j<=2n; j=j+2) //runs n times
    y=y+i;
 int s=0;
 for(i=1; i<=j; i++) // runs n times
 s++;

My Answer: O(n)

(f)

 int b=0;
 for(int i=0; i<n; i++) //runs n times
   for(int j=0; j<i*n; j++) //runs n^2 x n times? 
      b=b+5;

My Answer: O(n^4)

(g) Assume for simplicity that n