The answer is:
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"longshore drift" .
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The average velocity of Sandy is given by the total distance covered S divided by the total time taken t:

The total distance covered is

while the total time taken is 2 hours + half an hour (for the rest) + 1 hour and half, so

Therefore, the average velocity is
Explanation:
Fnet = Ft - Ff = 60N - 20N = 40N
a = Fnet / m = 40N / 25kg = 1.6m/s².
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