Let R be the car's rate
Call the time for the car to travel (in hours) = 210/R
Call the total time for the bike to travel (in hours) = 210/R - 1/3
Now.....the distance bike travels = its rate * its time = 120 * [ (210/R) - 1/3 ]
And the distance the bike travels to meet the car = 120* [ (210/R - 1/3] / 2 = 60[ (210/R - 1/3 ] = the same distance the bike travels back to "A"
So.....the distance the bike travels back to "A" / ts rate = some time (1)
And the distance the car travels after meeting the bike / its rate = the same time (2)
So.......equating (1) and (2) in algebraic terms, we have
( 210 - 60 [ 210/R - 1/3] ) / R = 60[ 210/R- 1/3 ] / 120
(210 - 60 [ 210/R - 1/3 ] ) / R = [ 210/ R - 1/3 ] / 2 cross-multiply
2(210 - 60[ 210/R - 1/3 ] = R [ 210/R - 1/3 ]
420 - 120 [ 210/R - 1/3 ] = 210 - R /3
420 - 25200/ R + 40 = 210 - R/3
460 - 25200/R = 210 - R/3
R/3 + 250 - 25200/R = 0 multiply though by R
(1/3)R^2 + 250R - 25200 = 0 multiply through by 3
R^2 + 750R - 75600 = 0 factoring, we get
(R - 90) (R + 840) = 0
Only the first factor set to 0 will provide a positive solution for R = 90 kmh = the car's rate