Remember that the total angle around a circle is 360 degrees. The question tells us that there is only 3/10 of the circle left, so to find the angle for the slices that are left, we have to multiply 360 by 3/10, which is the same as dividing 360 by 10 and then multiplying by 3.
360 × 3/10 = 360/10 × 3 = 36 x 3 = 108
The measure of the angle for the 3 slices that are left is 108 degrees.