Question 708515
Tough to think about this one.  I see this pathway so far:


How far back was the car (40 miles per hour) when the bicyclist reached the road sign?  We do not yet need the bicyclist's speed or distance - not yet - but we CAN determine how far back was the car.  40 miles per hour Multiplied by 1/2 hour (which is the 30 minutes) equals 20 miles back.  


Now imagine we have a number line to represent the road.  At zero is the road sign.  NEGATIVE 20 is where the car is when the bicycle is at zero.  

At time equals zero, car is at -20, bicycle is at 0 on the line.  
some time t passes.  At some particular t, car has gone {{{(-20)+40*t}}}, and bicycler has gone {{{0+10*t}}}.  We want these positions on the line to be equal; so my thought is now, 
{{{(-20)+40t=0+10t}}}, find {{{t}}}.