[問題]GWD23-26

[magic]

Q26:
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 ≤ t ≤ 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
答案是B
請問這題要怎麼算阿

[smallwhite]

我直接看：t=5時N(t)達最大值
所以兩點加五個小時就是七點

理論的做法好像是用拋物線（開口朝下）的解法吧...