Question 696355
Add their rates as if one car was standing still 
and the other was traveling at the sum of their speeds
Let {{{ t }}} = the time in hrs when they will be {{{ 150 }}} mi apart
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{{{ 150 = ( 60 + 30 )*t }}}
{{{ t = 150 / 90 }}}
{{{ t = 1.667 }}} hrs
Note that {{{ .667*60 = 40 }}}
They will be 150 mi apart in 1 hr and 40 min