the distance travelled by an object thrown upwards in t seconds is 30t - 4.9t square metres. after how much time would it fall down? at what all times would it be 20 metres above the ground?

Answer 1

When it falls down, the distance is zero
so 0 = 30t - 4.9t^2

t(30 - 4.9t) = 0
t = 0 , that would be at the beginning
or
t = 30/4.9 or appr. 6.12 seconds

2nd part:
20 = 30t - 4.9t^2
4.9t^2 - 30t + 20 = 0
t = (30 ± √508)/9.8
= .761 seconds, on its way up
or
= 5.36 seconds, on the way down