Question: Which numbers in $A$ are elements of both subsets $B$ and $C$?
$A = \{1, 2, 3, 4, 5, 6, 7, 8, 9, 10\}$
$B = \{3, 6, 9\}$
$C = \{7, 8, 9, 10\}$
The number(s) in all three sets is/are [blank box at bottom-center].
(Use a comma to separate answers as needed.)
1. **State the problem:** We need to find which numbers in set $A$ are elements of both subsets $B$ and $C$.
2. **Recall the definition:** The elements common to both $B$ and $C$ are found by the intersection $B \cap C$.
3. **Find the intersection $B \cap C$:**
$$B = \{3, 6, 9\}$$
$$C = \{7, 8, 9, 10\}$$
The only common element is $9$.
4. **Check if this element is in $A$:** Since $9 \in A$, it is indeed an element of all three sets.
5. **Final answer:** The number in all three sets is
$$9$$