Answer:
use the formula we know the distance is 1500 metres
put that in the formula
1500 = 5t^2 + 25t
Now we can factor out 5 as all the coefficients are multiples of 5
so
we get
300 = t^2 +5 t
now we solve for t using the quadratic factorization
t^2 +5t -300 = 0
which gives
(t - 15) =0
(t + 20 )= 0
so time cannot be negative hence
15 seconds is the answer