Answer
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Work Step by Step
a. The monitor screen is refreshed at regular intervals. It is as though the mouse cursor is being illuminated by a strobe light that flashes at regular intervals. The cursor is visible only when the screen is re-drawn, so it appears to jump.
b. If we see the cursor 15 times in 0.25 seconds, there are 14 intervals between the 15 screen refreshes. The interval is T = 0.25s/14, and the frequency is the reciprocal, or 14/0.25s. The refresh rate is 56Hz, which to one significant figure is 60 Hz.
