Description for Correct answer:
B alone can fill the tank in 40 mins., so if it works for 23 min it will fill \( \Large \frac{23}{40} \)th of the tank.

Remaining \( \Large \frac{17}{40} \)th of the tank has to be filled by A and B in

= \( \Large \frac{17}{40} \left[ \frac{45 \times 40}{45 + 40} \right] = \frac{17}{40} \times \frac{360}{17} \) = 9 min.