Answer:

The point of this puzzle turns on the fact that if the magic square were to be composed of whole numbers adding up 15 in all ways, the two must be placed in one of the corners. 
Otherwise fractions must be used, and these are supplied in the puzzle by the employment of sixpences and half-crowns. 
I give the arrangement requiring the fewest possible current English coins—fifteen. 
It will be seen that the amount in each corner is a fractional one, the sum required in the total being a whole number of shillings.