(b) 90 km
Speed downstream = (12 + 3) km/hr = 15 km/hr
Speed upstream = (12 – 3) km/hr = 9 km/hr
Let the distance between A and B be x km. Then
\(\frac{x}{15}+\frac{\frac{x}{2}}{9}=11⇒\frac{x}{15}+\frac{x}{18}=11\)
⇒ \(\frac{6x+5x}{90}=11⇒\frac{11x}{90}=11⇒x=90\) km.