Question 874286
More generally, two trains initially m miles apart start traveling toward each other simultaneously.  One is going r mph and the other s mph.  How much time to meet?


The unknown variable is t, in:
{{{rt+st=m}}}, because the sum of their distances traveled will be m when they meet, or collide.


{{{t(r+s)=m}}}
{{{highlight(t=m/(r+s))}}}.
Substitute the values to find that of t.