P, Q and R are three participants in a 10 km race. If P can give Q a start of 400 m and P can give R a start of 640 m, how may metres start can Q give R?
250 m
When P covers 10000 m, Q covers (10000 - 400) m = 9600 m and R covers (10000 - 640) m, or 9360 m
When Q covers 9600 m, R covers 9360 m
When Q covers 10000 m, R covers (93609600×10000)m=9750m
∴ Q can give R a start of (10000 - 9750) or 250 m