whileProblem.rb

valid = false
operations = []
formula = Formula.new(operations)
while !valid
  operations << Operation.new(
                  AVAILABLE_OPERATORS["Multiplication"],
                  Constant.new(Float((rand(1...1000)*10))/100),
                  Constant.new(Float(1 + rand(1...40)).round(2))
                )
  formula = Formula.new(operations)
  valid = (user_skill.level-10..user_skill.level+10).include?(formula.difficulty)
end

theory = "#{formula.operations[0].constant1.value*100}% van #{formula.operations[0].constant2.value}#{unit}"