Codechef ATM problem in ruby

atm.rb

#problem link : https://www.codechef.com/problems/HS08TEST

$stdin.each do |num|
    arr = num.strip.split(/\s+/).map(&:to_f)
    final = arr.last - arr.first - 0.50
   if (arr.first % 5) == 0 and arr.first < arr.last and arr.first > 0 and final >= 0
        puts ('%.2f' % final)
    else
        puts ('%.2f' % arr.last)
    end
end

=begin
 
 '%.2f' % value  <-- This returns 2 digits after decimal of the given value

=end