Aclock shows 4: 00.
what is the supplement of the smaller angle formed by the hands of this clock?

The answer would be 120 degrees.
We know a full circle is 360 degrees. And we know that a clock is a full circle. Therefore, a clock is 360 degrees.
There are 12 hours on the clock. We can divide 360 and 12 to find how many degrees each hour is. 360/12=30. 
Since 4:00 is four hours, we can multiply 30 (degrees per hour) by 4 (hours). 30x4=120, so the angle is 120 degrees.
Hope I helped! ;)