Can we do: 8C1 + 8C2 ? which is 8+ 28= 36 ? If no, why so?

GRE Quant Problem Solving
1

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Can we do: 8C1 + 8C2 ?

which is 8+ 28= 36 ?

If no, why so?

2

What’s your logic for 8c1 + 8c2 ?