From the equation of motion, we know,

Where s= displacement
u= initial velocity
a= gravitational force
t= time
Displacement is 0 since the ball comes back to the same point from where it was thrown.
A =
since the ball is thrown upwards.
Plug the known values into the equation.
=> 
Solving for u gives :
u= 16.67 m/ sec ....... equation (1)
At maximum height, final velocity i.e v is 0
Time take to reach the top = 

=> 
Solving for s we get
s= 14.16 m
Answer:
80 students
Step-by-step explanation:
12/80 is 0.15 or 15%
Answer:
4.5
Step-by-step explanation:
6x + 42 = 18x - 12 (alternate angles)
or, 18x - 6x = 42+12
or, 12x = 54
or, x = 54/12
x= 4.5