Example 3.8: Consider a particle projected vertically upwards with an initial velocity v₁ =20m/s from the top of a 15m building as shown in the figure below. How long does it take for the particle to reach to: A) level A, the initial point, B) level B 5m above the initial point, C) level C, 5m below the initial point and D) level D, the building bottom?​