According to a certain estimate, the depth \(N(t),\) in centimeters, of the water in a certain tank at \(t\) hours past

According to a certain estimate, the depth \(N(t),\) in centimeters, of the water in a certain tank at \(t\) hours past \(2:00\) in the morning is given by \(N(t)=-20(t-5)^2+500\) for \(0\le t\le 10.\) According to this estimate, at what time in the morning does the depth of the water in the tank reach its maximum?

a) \(5:30\)
b) \(7:00\)
c) \(7:30\)
d) \(8:00\)
e) \(9:00\)

Answer: B

Source: GMAT Paper Tests