Question 410876
Let {{{d}}} = distance 2nd train goes when they are 1420 km apart
The 1st train has gone {{{70*2 = 140}}} km when the 2nd train leaves.
Both trains travel for the same length of time when 2nd train leaves
2nd train:
{{{d[2] = 90*t}}}
1st train:
{{{d[1] = 70*t}}}
{{{d[1] + d[2] + 140 = 1420}}} km
By substitution:
{{{70t + 90t + 140 = 1420}}}
{{{160t = 1280}}}
{{{t = 8}}} hrs
The 1st train traveled {{{8 + 2 = 10}}} hrs
when they are 1420 km apart
check answer:
{{{d[1] = 70*8}}}
{{{d[1] = 560}}}
{{{d[2] = 90*8}}}
{{{d[2] = 720}}}
{{{d[1] + d[2] = 560 + 720}}}
{{{d[1] + d[2] = 1280}}}
{{{1280 + 140 = 1420}}}