Let aequals=startset 9 comma 10 comma 11 comma 12 comma 13 comma 15 endset{9,10,11,12,13,15}.
a. how many subsets does a have?
b. how many proper subsets does a have?

Let A = {9,10,11,12,13,15}
in this set number of elements is equal to six.
we have to find how many subsets and proper subsets A have.
for finding number of subsets, we use the formula 2 raise to the power n, where n is the number of elements in the set A which is 6, it means 2⁶ subsets
so, set A have 2⁶ = 64 subsets
and to find the proper subsets use the formula 2ⁿ - 1 = 2⁶ - 1 = 64 - 1 = 63
so, set A have 63 proper subsets.

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Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table