Bunuel
If the elevation above water level of a breaching wale is given by the expression −t^2 + 12t − 35 where t is the time, in seconds, after an observer begins watching, is the whale presently underwater?
(1) t < 8.
(2) t > 5.
A breaching whale is a whale leaping out of the water.
Elevation = −t^2 + 12t − 35
(1) t < 8
Let's notice what happens at exactly t = 8
Elevation = -(8)^2 + 12*8 - 35 = -3
It means the whale is under water at this time.
Put t = 7
Elevation = -(7)^2 + 12*7 - 35 = 0
So the whale is exactly at the surface at this time.
Put t = 6
Elevation = -(6)^2 + 12*6 - 35 = 1
The whale has a positive elevation at this time.
Let's find out what happens at t = 5
Elevation = -(5)^2 + 12*5 - 35 = 0
So the whale is exactly at the surface at this time.
We see what is happening here. The whale is exactly at the surface at t = 5, is out of water and then back in the water at t = 7.
Given that t < 8, the whale is underwater if t > 7 or t < 5. Not sufficient.
(2) t > 5
If t = 6, the whale is out of water but if t > 7, it is underwater. Not sufficient.
Using both, If t = 6, the whale is out of water but if t > 7, the whale is underwater. Not sufficient.
Answer (E)