Math word problem

Question

If it took 10 minutes to drain the bathtub water down to 450 gallons and than 30 minutes to drain it down to 150 gallons. How long will it take for it to be empty

Answer 1

150 gal / 30 min

5 gal / min

10 min (first drain) + 30 min (second drain) + 30 min (last drain)

How much time is that?