When they both pump, Engine #2 gains quantity over Engine #1 at the rate of 200 gallons per minute.
BUT ... when Engine #2 started up at 9:30, Engine #1 was already
(800 x 10 minutes) = 8,000 gallons
in the lead. Engine #2 has to gain 8,000 gallons just to pull even.
Since Engine #2 gains 200 gallons per minute, it will take him
(8,000) / (200) = 40 minutes
to pull even with Engine #1.
That moment will arrive at (9:30 + 40 minutes) = 10:10 AM .