I have 2 road rollers. These road rollers are joined end to end along the flat surfaces to form a combined longer road roller. What are the surfaces present in the new road roller?
2 flat surfaces, 1 curved surface
Consider the 2 road rollers (cylinders) as shown below.
Now let us join the 2 cylinders along the ends (flat surfaces).
In the above diagram we have joined the bottom end of top cylinder and top end of bottom cylinder. These 2 joined ends (flat surfaces) will no longer be visible.
Finally we are left with only 2 flat surfaces- The top surface of top cylinder and the bottom surface of bottom cylinder. Other than this there is one curved surface. The length of this curved surface is the sum of lengths of curved surfaces of 2 cylinders. The final combined object is represented below.
It is clear that this object has 1 curved surface and 2 flat surfaces.