In a 800 metre race, A defeated B by 15 seconds. If A's speed was 8 km/hr, the speed of B was -

A's speed
= 8 km/hr
= $$\left( {8 \times \frac{5}{{18}}} \right)$$   m/sec
= $$\frac{20}{9}$$ m/sec
Time taken by A to cover 800 m
= $$\left( {800 \times \frac{9}{{20}}} \right)$$   sec
= 360 sec
Time taken by B to cover 800 m
= (360 + 15) sec
= 375 sec
∴ Speed of B :
= $$\frac{800}{375}$$ m/sec
= $$\left( {\frac{{800}}{{375}} \times \frac{{18}}{5}} \right)$$   km/hr
= $$\frac{192}{25}$$  km/hr
= $$7\frac{17}{25}$$  km/hr