Puzzles
Turn 1 2 3 4 5 6 7 8 9, kept in that order, into exactly 100
Write the digits 1 through 9 in ascending order, then insert plus and minus signs between some of them (digits may be pushed together to form bigger numbers, but never reordered or repeated) so the result equals exactly 100. Dudeney's version asks for a solution using as few signs as possible.
Reveal the answer
One minimal solution is 123 − 45 − 67 + 89 = 100, using digits grouped into two- and three-digit numbers with just three signs. Dudeney set this as puzzle no. 94 in Amusements in Mathematics (1917); dozens of other sign arrangements also reach 100, but his 'fewest signs' framing is what makes finding the tightest one genuinely hard.
— Henry Ernest Dudeney, Amusements in Mathematics — 1917, puzzle no. 94