# No.4
list = [50, 2, 1, 9]
list.sort_by(&:to_s).reverse.join

# No.5
[*2..9].reduce(['1']) do
  |res, v| res.map { |r| ['', ' + ', ' - '].map { |oper| "#{r}#{oper}#{v}" } }.flatten
end.select { |v| eval(v) == 100 }