In adding quantities to individual rows, whether along the horizontal (x axis) or vertical (y) axis, you start from five pennies and end up with two rows: one row of two, along the y axis, and a row of three along the x axis. To solve it, you make one move and create to rows of three. To do this, you must move into the z axis – the axis of breadth and depth, the third dimension, and place the third penny in the x axis row by placing it on top of the first penny in each row, thus, you create two rows of three with one move.
To resolve this, move penny labeled 3 on the x axis onto the top of the first penny, which is the first penny and beginning of axes.
If this was obvious to you, enjoy your Richard Feynmann lecture, nerd.
Notes: The idea behind the puzzle is to test spatial awareness, to demonstrate the z axis in mathematics, and (hopefully) teach students to think in three dimensions. The problem is in the assumption that the solution must be limited to the horizontal and vertical axes when the only way to solve the problem in the context of making one move (to move one penny) to turn two rows of different quantities into rows with equal quantities, from an x axis row of three and a y axis row of two, by removing the third penny in the x axis and placing it on the first penny in the z axis makes each row contain three pennies, making them of equal quantity, that of 3.