For storing a 3-D array in column major order, we'll store planes first, then columns and finally rows.
Consider a 3-D array A[M][N][P] stored as A[d1....dn1][d2...dn2][d3...dn3]
where d1 = lower bound of dimension 1
dn1 = upper bound of dimension 1
d2 = lower bound of dimension 2
dn2 = upper bound of dimension 2
d3 = lower bound of dimension 3
dn3 = upper bound of dimension 3
Now, an element at location A[i][j][k] in CMO will be found at
Location(A[i][j][k]) = Base address + { (i-d1)*(number of rows)*(number of colums) + (k-d3)* number of rows + (j-d2) } * size of one element
Now, let's do your question!
Assuming one element takes 4 bytes,
Location of A[20][20][30] = 10 + { (20-1)*30*40 + (30-1)*30 + (20-1) }*4 = 94766
