Submitted following solution, but WA on 1st test. I tried very different inputs and works fine actually.

def get_input_number
        gets.chomp.to_f
end

def calculate_conductors(low_percent, high_percent)
        (2..10000).each do |citizens|
                konduktors = (low_percent*citizens).ceil
                if (low_percent*citizens < konduktors) && (konduktors < high_percent*citizens)
                        return citizens
                end
        end
end

def prepare_input_and_go
        lower_border = get_input_number/100
        upper_border = get_input_number/100
        puts calculate_conductors(lower_border, upper_border)
end

prepare_input_and_go