A currency curiosity discovered by Lewis Carroll:

Write down any number of pounds not more than 12, any number of shillings under 20, and any number of pence under 12. Under the pounds figure write the number of pence, under the shillings the number of shillings, and under the pence the number of pounds, thus reversing the line.

Subtract. [If you need to make exchanges, 1 pound = 20 shillings = 240 pence.]

Reverse the line again.

Add.

“Answer, 12 pounds 18 shillings 11 pence, whatever numbers may have been selected.”