Hi, I'm trying to combinatorially prove the following identity:

On the right side, is the number of ways we can choose a -element subset from a -element set, while on the left side there is the sum of ways we can select an -element subset from a -element, -element,..., -element set...

How can we prove that the numbers on both sides of the equation are actually identical?

I'd be really grateful for any help.

Thanks!