Up to 100
by Henry E. Dudeney and Martin Gardner
The nine digits 1 through 9 are written out in a row in ascending order as
shown in the upper row of the illustration. The object is to insert
between the digits, without changing their positions in the row, several arithmetical signs so that to get exactly a hundred as the
result. From the four arithmetical signs you can use only plus (+) and
minus (-). The solution with six signs is shown as an example in the lower
right corner of the illustration. But can you find a solution where three
signs are used only?
No less interesting and harder is the "reverse" puzzle when the digits are
in descending order as shown in the lower row of the illustration. The
same rules as to the previous puzzle are applied, except that this time
the four signs (again + and - only) have to be used. Can you find this
solution as well?