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For two sets $\text{A}$ and $\text{B}$, let $\text{A} \triangle \text{B}$ denote the set of elements which belong to $\text{A}$ or $\text{B}$ but not both. If $\text{P} = \{1,2,3,4\}, \text{Q} = \{2,3,5,6\}, \text{R} = \{1,3,7,8,9\}, \text{S} = \{2,4,9,10\},$ then the number of elements in $(\text{P} \triangle \text{Q}) \triangle (\text{R}\triangle \text{S})$ is

1. $9$
2. $7$
3. $6$
4. $8$

Given that,

• $\text{T} = \{ 1,2,3,4\}$
• $\text{Q} = \{ 2,3,5,6 \}$
• $\text{R} = \{ 1,3,7,8,9\}$
• $\text{S} = \{ 2,4,9,10\}$

$\boxed{ \text{A} \triangle \text{B} = ( \text{A} \cup \text{B}) – ( \text{A} \cap \text{B})}$

$\boxed {\text{A} \triangle \text{B} = ({ \text{A} – \text{B}) \cup (\text{B} – \text{A})}}$

Now, $\text{P} \triangle \text{Q} = \{ 1,2,3,4\} \triangle \{ 2,3,5,6\}$

$\Rightarrow \text{P} \triangle \text{Q} = \{ 1, 4, 5,6 \}$

And, $\text{R} \triangle \text{S} = \{ 1,3,7,8,9 \} \triangle \{ 2,4,9,10 \}$

$\Rightarrow \text{R} \triangle \text{S} = \{ 1,2,3,4,7,8,10 \}$

Thus, $( \text{P} \triangle \text{Q}) \triangle ( \text{R} \triangle \text{S}) = \{ 1,4,5,6 \} \triangle \{ 1,2,3,4,7,8,10 \}$

$\Rightarrow ( \text{P} \triangle \text{Q}) \triangle ( \text{R} \triangle \text{S}) = \{ 2,3,5,6,7,8,10 \}$

$\therefore$ The number of elements in $( \text{P} \triangle \text{Q}) \triangle (\text{R} \triangle \text{S})$ is $7.$

Correct Answer $: \text{B}$

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