A particle is projected vertically upwards with speed 40 m s^-1 alongside a building of height h m.

(a) Given that the particle is above the level of the top of the building for 4 s, find h.

(b) One second after the first particle is projected, a second particle is projected vertically upwards from the top of the building with speed 20 m s^-1.

Denoting the time after projection of the first particle by t s, find the value of t for which the two particles are at the same height above the ground.