Answer: The projectile reaches its maximum height after 20 seconds.
Step-by-step explanation:
Given the function:
You can following formula (which gives the x -coordinate of the vertex of the parabola) in order to find after how many seconds the projectile takes to reach its maximum height:
In this case you can identify that the values of "a" and "b" are:
Then, substituting values into the formula, you get the following result:
Therefore,based on this, the conclusion is: The projectile reaches its maximum height after 20 seconds.