Each minute on a clock face corresponds to an angle of
360/60 = 6 deg.
So in 18 minutes, the minute hand advances by 18 * 6 = 108 deg.
The hour hand advances at 1/12 of the rate of the minute hand, so in 18 minutes it advances by
108/12 = 9 deg.
In 18 minutes, the minute hand is ahead of the hour hand by
(108 - 9 - 20)
= 79 deg.