The ratio of speed of A and B is 7 : 10 and A loses the race by 210 m, then find the length of the race.

700 m

The time is same for A and B. 

Therefore, Distance ∝ Speed,

                  A          :           B

Speed         7          :           10

Distance      7          :           10          Difference = 3R

3R = 210

1R = 70

The length of race = distance covered by B = 10 × 70 = 700 m