Question 1068565
I will say that {{{ h(0) = 400 }}} m
where {{{ h(t) }}} is the height in meters
{{{ h(t) = -(1/2)*g*t^2 + h(0) }}}
{{{ h(t) = -(1/2)*9.8*t^2 + 400 }}}
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I want to find {{{ t }}} when {{{ h(t) = 0 }}}
( the bottom of the well )
{{{ 0 = -4.9t^2 + 400 }}}
{{{ 4.9t^2 = 400 }}}
{{{ t^2 = 81.6327 }}}
{{{ t = 9.035 }}} sec
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After {{{ 9.035 }}} sec, the sound has to travel 
{{{ 400 }}} m to Joe's ears
[ time in sec ] = [ distance in meters ] / [ speed in m/sec ]
{{{ 400/343 = 1.166 }}} sec
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Add the 2 computed times:
{{{ 9.035 + 1.166 = 10.2 }}} sec
Joe hears the sound of the rock
hitting the water in 10.2 sec
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check my math