Explanation:
don't panic. just breathe and then use common sense.
1.
1 math question in 1/200 second
2 minutes have 2×60 seconds = 120 seconds.
each of the seconds has 200 1/200 seconds.
so, in 2 minutes we have
120 × 200 time units to finish math questions.
in each time unit 1 math question can be answered.
120×200 = 24,000
the computer can answer 24,000 math questions in 2 minutes.
2.
it can run for 15 minutes.
15 minutes have
15×60 = 900 seconds (as every minute has 60 seconds)
it can vacuum 1/2 ft² in 1 seconds.
in 900 seconds it can then vacuum 900×1/2 = 450 ft².
so, yes, it can clean 400 ft² without recharging.