Question 996268
You can think of this as one person 
standing still and the other traveling
at the sum of their speeds
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Let {{{ t }}} = time in hrs when they meet
{{{ 4 = ( 3 + 9 )*t }}}
{{{ 4 = 12t }}}
{{{ t = 1/3 }}}
Convert hrs to min
{{{ (1/3)*60 = 20 }}}
They will meet in 20 min
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check:
Let {{{ d }}} = distance the walking man
travels in 20 min
{{{ 4 - d }}} = distance friend travels
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Equation for walker:
{{{ d = 3t }}}
{{{ d = 3*(1/3) }}}
{{{ d = 1 }}} mi
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Equation for biker:
{{{ 4 - d = 9*(1/3) }}}
{{{ 4 - d = 3 }}}
{{{ d = 1 }}} mi
OK