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Hi Abraham, The problem states
I solved for the cosine when the tide is 7 feet and got
I then sketched y = cos(x) for x between 0 and 2 Pi radians.
cos(x) = 1/2 when x = 2 Pi/3 radians and at x = 4 Pi/3 radians. Thus cos(pi/6 t) is less than 1/2 for Pi/6 t between 2 Pi/3 and 4 Pi/3.
Thus the tide is less than 7 feet for the 4 hours between 4 and 8, and hence the tide is at least 7 feet for 8 hours. Penny 

