To find the number of chew toys Rex has, you can express the relationship in the equation:
\( r = \frac{1}{2} \times 12 \)
So the correct operation to fill in the blank is multiplication.
Thus, your one-step equation would be:
\( r = \frac{1}{2} \times 12 \)
In terms of filling in the blank with an operation, the answer is:
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Rex has 6 chew toys.