The variation statement tells you that when the radius is multiplied by 5, the volume is multiplied by 5² = 25. When the height is multiplied by 1/2, the volume is multiplied by 1/2.
Both of these variations together will multiply the volume by 25·(1/2) = 25/2.
So, the revised volume is (2π in³)·25/2 = 25π in³.