The reason for the equations x + 2y = 5 and 4x + 8y = 20 to have infinite solutions is
The graph of both the equations is the same line
On sketching the graph, we get the same line. In the line 4x + 8y = 20, if we take the common factor 4 out, we get x + 2y = 5 which is same as the other line.
So, these lines are coincident and therefore, have infinite points of intersections (solutions).