This also is an adaptation of a very old problem. An assistant is told that he can accept IOU's from members of the audience for any amount not exceeding one shilling (12 pennies, which you can think of as 12 cents). He is given one shilling in coppers for "change" which change must be used to bring the value of any IOU below one shilling up to that amount. Thus, if someone hands in an IOU for 9d (9 cents). the assistant gives him 3d. (3 cents) change from his shilling's-worth of coppers. The assistant cannot accept any lOUs involving change unless he has sufficient cash in hand to make up the amount to 1/- (1 shilling, or 12 cents). Whilst the money exchanges are being effected the mentalist is either out of the room or out of sight and hearing of the transactions that are being conducted. When all is ready the mentalist appears on the scene. If there is any change left this is handed to him. He is told the number of IOUs accepted and immediately reveals the total face value of these IOUs.

Solution: The IOUs should be written on small slips of paper. Suppose there are five IOUs with 3d. (3 cents) change left over from the original shilling. The mentalist immediately reveals that the total amount of the IOUs is 4/3d.(4 shillings and 3 pennies, or 51 cents), i.e. the total value of the IOUs is always one shilling less, in shillings, than the number of vouchers, plus the amount of change left over. If there were four IOUs and 7d. (7 cents) change the total value of the vouchers would be 3/7d. (3 shillings and 7 pennies, or 43 cents).

In this example 1/- (1 shilling) has been chosen as the maximum value of the IOUs that would be accepted. The solution would be much less likely to be detected if the maximum was increased to, say 1/3d. (1 shilling and 3 pennies, or 15 cents). In this case fifteen pence would be handed to the assistant who would be told to give change for any IOU below i/3d. When the mentalist returns to the room, suppose that 2d. (2 cents) change is handed to him and he is told that the assistant has six IOUs.

He would answer, immediately, that the value of these IOUs was 6/5d. (6 shillings and 5 pennies, or 77 cents), i.e. 6 (number of IOUs) minus 1 (always 1) = 5; 5 x 15 (Limit established at beginning of trick) = 75 plus 2 (The 2 is the change, giving a total of 77 cents) = 6/5d. (77 cents broken up into shillings and pennies)