Counting Formula Clarification (Groups/Cosets)

[EV33]

Homework Statement

G is a group. H is a subgroup.
lHl= order of H
lGl=order of G
[G]=Number of cosets

Counting Formula
lGl = lHl [G]


I have a question of clarification about this formula. My book says that [G]=number of cosets.

The problem is that at this point in my book they haven't defined right cosests yet so I wasn't sure if
[G]= The sum of the rights cosests and left cosets?

or

[G]= Just the left cosets?


Thank you.

 

[HallsofIvy]

Just the number of left cosets which is equal to the number of right cosets.

 

[EV33]

Thank you, HallsofIvy.