Question 682133
I started the answer with -16t^2+16t+410=90, then subtracted 90 to get -16t^2+16t+320=0 and then perform the quadratic equation to get the answer. Is this the right start?
Yes, applying the "quadratic equation" is one approach.
But, you could have "factored" it (an easier approach):
-16t^2+16t+320=0
dividing both sides by -16:
t^2-t-20=0
factoring:
(t+4)(t-5) = 0
t = {-4, 5}
throw away the negative solution (extraneous) leaving:
t = 5 seconds
.