Description for Correct answer:
Between 4 and 5 o 'clock the hands of the clock will be at right angle once. When the two hands are at right angles, they are 15 minutes space apart. When the minutes hand is 15 minute space ahead of the hour hand.

To this the minute hand will have to gain \( \Large \left(20+15\right) \) minutes

= 35 minutes space.

Now, 55 minutes space are gained in 60 minutes.

35 minutes spaces will be gained in \( \Large \left(\frac{60}{55} \times 35\right) \) minutes

= \( \Large 38\frac{2}{11} \) minutes

So they are at right angle at \( \Large 38\frac{2}{11} \) minutes past 4.