A.25 kg baseball is thrown upward at a speed of 30 m/s. what is the maximum height the ball reaches?

Since gravity slows an upward-moving object by 9.8 m/s every second,
the baseball reaches its greatest height in (30/9.8) = 3.06 seconds.
At that instant, its speed is zero.

The baseball's average speed from toss to peak is

             (1/2) (30 + 0)  =  15 m/s .

At an average speed of 15 m/s for 3.06 seconds,
the ball rises    (15 x 3.06)  =  45.9 meters.

(We've known for about 500 years now that if air resistance
can be ignored, then the mass makes no difference.
In this solution, I ignored air resistance, because I don't
know any of the 15 or 20 other things that I'd need to know
in order to include it, and if I had them, I wouldn't know how to
use them anyway.  That's why we always ignore air resistance
in problems like this, unless we are aeronautical engineers and
have access to some serious electronic computing power.)