Ratio of times taken by A and B = 1/2 : 1/3
Suppose B takes Tb min. Then A takes (Tb + 10) min.
∴ (Tb + 10 ) : Tb = 1/2: 1/3
⇒ (Tb + 10)/Tb = 3 / 2
⇒ 2Tb + 20 = 3Tb
∴ Tb = 20
∴ Time taken by A = 20 +10
= 30 minute
If A had walked at double speed
Req . time = 30/2
=15 minute