DouKaku?

n = (1..9).to_a
op = ["",".0/","+","-","*"]
for i in 0..((5**8)-1)
  a = n.zip(("%08d" % i.to_s(5)).chars.map{|j|op[j.to_i]}).to_s
  puts a.tr("*/+-","×÷＋－").concat("＝#{eval(a)}").gsub(/\.0/,"") if eval(a) == 100.0
end

# => 123＋45－67＋8－9＝100
# => 123＋4－5＋67－89＝100
# => 123＋4×5－6×7＋8－9＝100
# => 123－45－67＋89＝100
# => 123－4－5－6－7＋8－9＝100
# => 12÷3÷4＋5×6＋78－9＝100
# => 12÷3＋4×5－6－7＋89＝100
# => 12÷3＋4×5×6－7－8－9＝100
# => 12÷3＋4×5×6×7÷8－9＝100
# => 12＋34＋5×6＋7＋8＋9＝100
#...101個表示。