Question 235444
Each change requires a different
equation because a change will make 
the graph of {{{d = r*t}}} a different
straight line
1st part of trip:
{{{200 = x*t}}}
2nd part of trip:
{{{240 = (x - 20)*t}}}
This is 2 equations and 2 unknowns, so
it's solvable
My tip for word problems is: A small amount of time
spent really thinking about what's going on in
general is worth doing a hundred problems.