The speed v ms/s of a particle travelling from A to B, at time t s after leaving A, is given by v=10t-t^2.The particle starts from rest at A and comes to rest at B. Show that the particle has a speed of 5ms/s or greater for exactly 4sqrt5 s.

Paper answer:

v=5 => t1=5-2sqrt5, t2= 5+2sqrt5

Therefore, t2-t1=4sqrt5

I dun understand why is it like dat, can someone explain it to me??