Answer:
Diana must take 22 rotations to throw at least 400 strikes
Explanation:
Diana needs to throw more than 400 pitches in the strike zone and each time she rotates to the mound, she throws 20 pitches. We know she misses a total of 20 pitches on that practice. She still needs to throw more than 400 strikes, so the total pitches she needs is at least 401+20=421. Recall she needs to throw more than 400 strikes.
In rotations of 20 pitches she will need 421/20=21.05 rotations, but we need to round up. Thus the answer is
Diana must take 22 rotations to throw at least 400 strikes