Question 1137306
Then distance between them is {{{ 8900 - 4400 = 4500 }}} ft
Add their rates of climbing
{{{ 600 + 900 = 1500 }}} ft/hr
Let {{{ t }}} = time in hrs until they pass each other
{{{ 4500 = 1500*t }}}
{{{ t = 3 }}} hrs
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check:
{{{ d[1] = 600*3 }}}
{{{ d[1] = 1800 }}} ft
and
{{{ d[2] = 900*3 }}}
{{{ d[2] = 2700 }}} ft
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{{{ 1800 + 2700 = 4500 }}} ft 
OK