Pls come an instructor, thank you

tracyyahoo wrote:Acoording 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)+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

Could someone explain in details , tahnk you.

OA is B not A not C ok~~~~ I chose C I have my reasons, pls explain why A???

sanju09 wrote:

tracyyahoo wrote:Acoording 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)+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

Could someone explain in details , tahnk you.

OA is B not A not C ok~~~~ I chose C I have my reasons, pls explain why A???

Please recheck your question, because the depth function N (t) = -20(t – 5) + 500 for 0 ≤ t ≤ 10 suggests that N (t) will be maximum when the expression -20(t – 5) will attain its maximum value within the interval 0 ≤ t ≤ 10, and that happens at t = 0 or at 2:00 in the morning, which is none of the choices.

tracyyahoo wrote: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

GMATGuruNY wrote:

tracyyahoo wrote: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

Depth = -20(t-5)²+500.

-20(t-5)² ≤ 0.
Thus, the maximum depth will occur when -20(t-5)² = 0.
-20(t-5)² = 0 when t=5.

2am + 5 hours = 7am.

The correct answer is B.

seema19 wrote:
@GMATGuruNY - why should -20(t-5)² <= 0 ???