In a 100m race A runs at a speed of 9 km/ hr. He gives a start of 10m to B and still defeats him by 10 sec. Find speed of B (in m/sec) .

1.8

Total length of race = 100m 

Speed of A = 9 km/hr = 9 × \(\frac{5}{18}\) m/sec = \(\frac{5}{2}\) m/sec

Time taken by A to complete the race = \(\frac{100}{\frac{5}{2}}\) = 40 sec 

According to question , 

B takes 10sec more to run the whole race 

∴ Time taken by B to cover the race = 50 sec 

As A gives 10m start to B of 10m 

∴ Speed of B =  \(\frac{90}{50}\) m/sec = 1.8 m/sec