I come up v this solution:

$10 choose 1 + 10 choose 3 + 10 choose 5+ 10 choose 7 + 10 choose 9 =512$.

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But the solutions claims it"s $2^9=512$, which i don"t kinda get. Aren"t you counting the variety of subsets with, for example, $2$ elements? (I understand it"s the exact same solution, it simply doesn"t make sense to me).

Can someone describe me why this is correct and what"s the thinking behind this?

One means to see this combinatorially is to consider the $10$ elements in some order. With each the the very first $9$ elements, you have a complimentary choice the either consisting of it or not consisting of it, because that $2$ options each. However, there"s no an option for the $10$th element, due to the fact that to ensure the complete # of aspects in the set is odd, this aspect must be had if the full # so far is even, and not contained if the total # is odd. Thus, since there space $2$ options for each of the very first $9$ elements, this way there space $2^9$ choices overall.

The number of sets with odd strength is the very same as the variety of sets with also power. So, since the number of all (sub)sets is $2^n$ the price is $2^n-1$.

Start through a ten element collection and take it one element out, calling it $x$. Us now have a nine aspect set. For each subset, $S$, of the nine facet set, either$S$ has an odd number of elements, so need to be counted together a subset of the ten element set containing an odd number of elements.$x cup S$ has an odd number of elements, so should be counted as a subset that the ten element set containing an odd variety of elements.

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There room $2^9$ subset the a nine facet set. Us have presented that every among these have the right to be made into a subset the a ten element collection having an odd variety of elements.

What we have not shown is the every subset the a ten element set having one odd variety of elements has actually been created in this way. So, if a subset the a ten element collection having one odd variety of elements does no contain $x$, that is a subset the the nine aspect set, for this reason is contained in the counting above. If a subset the a ten element collection having one odd variety of elements does contain $x$, deleting $x$ from the subset yields a subset the the ripe element set having an even number of elements, for this reason is had in the counting above.