A simple problem

20(360÷60)=20*6

In $60$ minutes, the minute hand browses $360^\circ$ . Since the angular velocity of the minute hand is constant, in $20$ minutes, the minute hand browses $\boxed{120}^\circ$ .

$\blacksquare$

120, because 30 mins. is 180 and are six numbers in the middle of a clock, so, 180/6 = 30 and 180-30 is 150, and 150° is on 25 minutes. Also, 150-30 is 120. 120 is the answer because 150 is 25 minutes and 120 is 20 minutes.