Question 62553
AN OBJECT IS PROJECTED UPWARD FROM THE TOP OF A TOWER.  ITS DISTANCE IN FEET ABOVE THE GROUND AFTER t SECONDS IS GIVEN BY s(t)=-16t^2+64t+80.  HOW MANY SECONDS WILL IT TAKE TO REACH THE GROUND?
s(t)=0 when the object is on the ground, so set the problem = 0 factor and solve.
{{{0=-16t^2+64t+80}}}
{{{0=-16(t^2-4t-5)}}}
{{{0/-16=-16(t^2-4t-5)/-16}}}
{{{0=t^2-4t-5}}}
{{{0=(t-5)(t+1)}}}
t-5=0 and t+1=0
t=5 and t=-1
ignore the negative value it it's not pertinent.
t=5 s for the object to hit the ground.
Happy Calculating!!!!