Perspective view of two identical hollow cylinders is shown below. Assume the outer diameter and length of each cylinder are equal. The two cylinders intersect each other at 90 degrees to form a union, such that their centroids (centre of axis) coincide. How many visible surfaces will the resultant union have?
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For intersecting hollow shapes, first count the untouched flat end caps, then use symmetry to break down the outer curved walls and inner passages into simple quadrants.
Step 1: Count one cylinder. A hollow cylinder has 2 annular end faces plus outer and inner curved surfaces. Step 2: Subtract overlaps. The 90-degree union removes the intersecting curved and annular patches, leaving 20 visible surfaces. \[ \boxed{\text{Option 4}} \]
