2021-008

The space exploration agency AES is testing a new type of submarine, called XY, which can fly out from under the ocean and reach space. In its first test, there were failures that caused the first XY prototype to have a total loss of its engines, stopping its ascent at a certain time and starting to fall afterwards. The trajectory of the prototype in the time interval $0\le t\le 20$ is given by $f(t)=56+5t-(t-8{)}^2$.

What is the exact time $t$ when the XY prototype stopped going up towards space?

1. 20/2.
2. 21/2.
3. 2/21.
4. -21/2.
5. None of the above.

Original idea by: Adolfo Schneider