ext2 finding the block that contain the group descriptor from the inode number

Question

i found many source code use this method to find the group descriptor of the group that associated with that inode number

algorithm which is as follow :

1. group number = inode number / how many inode per group

2. group descriptor block = group number / number of group descriptors per block

the operation #2 i can't understand .

if there is an alternative method to find the block that contain the group descriptor block ,can you explain it .