split_sum.rb

#Pretend the sum is a big decimal and we want to conditionally append a 0 to replace
#the second dec place because ruby can chop the sec dec point

sum = "540.2"
  def dec_split_sum(sum)
		# Convert Sum to a string and then split it at the decimal point.
		sum = sum.to_s.split('.')
    if sum[1].length == 1
      puts sum[0] + '.' + sum[1] + '0'
    else
      puts sum[0] + '.' + sum[1]
    end
  end

	def dec_round_sum(sum)
		# Not working example, doesn't add a 0
		puts sum = sum.to_f.round(2)
	end

	def dec_format_sum(sum)
		puts format("%.2f",sum)
	end
dec_split_sum(sum)
dec_round_sum(sum)
dec_format_sum(sum)

Consider the big decimal in your dec_round_sum method. The big decimal is basically a float so calling .to_f really doesn't do much.

But the format_sum is cool but in this instance we need to actually split dollars and cents to be used separately. At least in my use case. :)