Calculate the volume of the cylindrical mug. I can't help you with that because "a base of 7 cm" as part of a description of a cylinder makes no sense. Did you mean 7 square cm as the area of the base, or did you mean 7 cm is the diameter of the base, or did you mean the base has a radius of 7 cm?
The formula for the volume of a cylinder is:
Where is the area of the base of the cylinder, is the height of the cylinder, and is the radius of the cylinder base.
Once you have calculated the volume of the cylinder, subtract to account for the volume taken up by 20 ice cubes each of which having volume.
John