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Or there is another possible solution: you know that the plane is travelling at 350 knots so in 60 minutes it's travelling 350 nautical miles; the half of 350 is 175 (30 minutes is an half hour, big surprise) so just do some quick math and voila... As I know there are 4 possible answers so you do not even have to do very exact calculates.
It's true for the second example too: speed=911 knots for 2 hours=1822 miles + 22 minutes (which is 1/3 hour, so the 30% of 911 is around 303-304) so all you have to do in your head is not too complicated: adding them! And you will be very happy to finish the math part before everybody else.
And everybody has to watch out: there might be funny switches between nautical and "normal" miles.