QR question with distance, time and speed

Hello guys,
Valerie drives on the highway at a speed of 60 mph for 15 minutes and then 45 mph on the local roads for 20 minutes. what is her Average velocity ?

I understand that avg velocity = total dist / time.

But why can't we take the avg of both the speed ? - (speed 1 + speed 2) / time ?

Thanks

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QR question with distance, time and speed