We have to find the number of options for setting pair $(a,b)$ under the terms: $a ⊆ b ⊆\{1, 2,\ldots, n\}$ Means, they are both subsets of $\{1, 2,\ldots, n\}$ and $a⊆ b$.
I was thinking to handle the $b$ coordinate first and by that, to handle the $a$ coordinate. But, what are the number of options for $b$?
tahnx.