Question 575097
You can think of one train as standing still and
the other approaching at the sum of their speeds.
I will say the slower one is standing still.
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The faster one approaches at {{{ 100 + 80 = 180 }}} km/hr
{{{ 90 = 180t }}}
{{{ t = .5 }}} hrs
They meet in a half hour
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Now, switching back to the original situation:
Equation for faster train:
{{{ d[1] = 100t }}}
{{{ d[1] = 100*.5 }}}
{{{ d[1] = 50 }}}
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Equation for the slower train:
{{{ d[2] = 80*t }}}
{{{ d[2] = 80*.5 }}}
{{{ d[2] = 40 }}}
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The faster train has gone 50 km when they meet