This problem involves the use of a kinematic equation since it involves the motion of an object. The equation for the height of the object is given as:
s = -16*t^2+v_o*t
Also, the initial velocity, v_o, was also said to be equal to 128 ft/s.
For the first question, the time(s) that the projectile will reach a height of 240 ft when v_o is 128 feet per second, can be solved using the given equation and the quadratic formula. The resulting quadratic equation is then -16*t^2 + 128*t -240 =0, where a =16, b =128, and c =-240. The quadratic formula is equal to [-b ± sqrt(b^2 -4ac)]/2a. This gives two answers t = 3 seconds and t = 5 seconds. This might be because the projectile has a parabolic path, thus, it reaches the height of 240 ft, before and after it reaches a peak.
For the second question, the time it takes for the projectile to reach the ground is obtained by setting the distance, s, equal to zero. In this case, t = 8 seconds.