Question 959050
You can think of this a one train ( either one )
moving at the sum of their speeds, and the
other train standing still
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Let {{{ t }}} = time in hours when they are
{{{ 620 }}} km apart
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{{{ 620 = ( 85 + 70 )*t }}}
{{{ 620 = 155t }}}
{{{ t = 4 }}}
In 4 hrs they will be 620 km apart
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check:
Equation for southbound train:
{{{ d[1] = 70*4 }}}
{{{ d[1] = 280 }}}
Equation for northbound train:
{{{ d[2] = 85*4 }}}
{{{ d[2] = 340 }}}
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{{{ d[1] + d[2] = 280 + 340 }}}
{{{ d[1] + d[2] = 620 }}}
OK