As a ching-chang is worth twopence and four-fifteenths of a ching-chang,
the remaining eleven-fifteenths of a ching-chang must be worth twopence.
Therefore eleven ching-changs are worth exactly thirty pence, or half a
crown. Now, the exchange must be made with seven round-holed coins and
one square-holed coin. Thus it will be seen that 7 round-holed coins are
worth seven-elevenths of 15 ching-changs, and 1 square-holed coin is
worth one-eleventh of 16 ching-changs--that is, 77 rounds equal 105
ching-changs and 11 squares equal 16 ching-changs. Therefore 77 rounds
added to 11 squares equal 121 ching-changs; or 7 rounds and 1 square
equal 11 ching-changs, or its equivalent, half a crown. This is more
simple in practice than it looks here.