Answer:
1.079 seconds
Step-by-step explanation:
You want to find t when H(t) = 15.
15 = -16t^2 +8t +25
10/16 = t^2 -1/2t . . . . . . subtract 24, divide by -16
10/16 + 1/16 = t^2 -1/2t +(1/4)^2 . . . . . add (1/4)^2 to complete the square
11/16 = (t -1/4)^2
t -1/4 = √(11/16) . . . . . take the positive square root
t = 1/4 +(√11)/4 = (1+√11)/4 . . . . use a common denominator
You would be 15 ft above the water at t = (1+√11)/4 ≈ 1.079 seconds.
