A rock was thrown upward with an initial velocity of 24 feet per second. The rock’s height t seconds after it was thrown upward is given by the function f(x)=−16x2+24x+4755
f
(
x
)
=
−
16
x
2
+
24
x
+
4755
. Four students were trying to guess how long it would take the rock to reach the ground: Kelly guessed that it would take 15 seconds, David guessed 18 seconds, Shelly guessed 20 seconds, and Dan guessed 25 seconds. Who had the best guess?