Answer
a. 3.091s, 0.990s
b. 4.082s
c. It does not reach 100m
Work Step by Step
$ s=-4.9t^2+20t $
$-4.9t^2+20t=15$
$-4.9t^2+20t-15=0$
$t=\frac{20+-\sqrt(20^2-4(-4.9*-15))}{2*-4.9}$
t= 3.091, 0.990
$-4.9t^2+20t=0$
$-t(4.9t-20)=0$
$4.9t-20=0$
$t=\frac{20}{4.9}$
t=0, 4.082
$-4.9t^2+20=100$
$-4.9t^2+20-100=0$
The discriminant of this equation is equal to $b^2-4ac$,
If this is less than 0 then there are no solutions.
$20^2-4(-4.9*-100)=-1560$
Because of this the object never reaches 100m.
