A can finish 1000m race in 3 min 10 sec. While B can finish the same race in 3 min 20 sec. By what distance A will defeat B in a race of 1000 m.

50m

According to question , 

A runs a distance of 1000m in 3 minutes 10sec and B runs same distance in  3 minutes 20sec . 

A  beats B by 10 sec 

Distance travelled by B in  3 minutes 20sec ( = 200 sec ) = 1000m 

Distance travelled by B in 10 sec = \(\frac{1000\;×\;10}{200}\) = 50m