Elizabeth has to manipulate twenty-four large
jampots in as many pigeon-holes.

She wants to get them in correct numerical order : 1,
2, 3, 4, 5, 6 on the top shelf, 7, 8, 9, 10, 11, 12 on the next shelf,
and so on.

Now, if she always takes one pot in the right hand and another in the
left and makes them change places, how many of these interchanges will
be necessary to get all the jampots in proper order?

She would naturally first change the 1 and the 3,
then the 2 and the 3, when she would have the first three pots in their
places.

How would you advise her to go on then?