The answer for your question is 3.5 dogs.
Reason for this: The reason why it is 3.5 dogs is because 21 is the total number of dogs and cats, but then, there are 6 times as many dogs as cats, which means that we have to multiply.
3 * 6 is 18, but 18 is not the total number (it is less than the total number), so 3 is wrong.
4 * 6 is 24, but 24 is not the total number (it is greater than the total number), so 4 is nt it either.
But what about 3.5?
3.5 * 6 is 21, and the total number is 21, so 3.5 is the answer for your question!
Hope this helped!
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