I am newbie in Ruby, I cannot find the bug on my financial program.
The question is related to local variable in block.
Here is a quick example:

a = 5000
r = 0.06
p = 100 
(1..5).each {|i|
  puts "#{i}\t\#{a}\t#{a*r}\t#{p}" # <-- the #{a} is not print
correctly, why?
  a -= (p - a*r)
}

Thanks for help.

-------------------------

Below is my financial program, skip it if you are not interesting.
In fact, I want to know how the bank calcule the payment per year for
a loan.

Below is my quick and throw away program. You can play it.
BTW: If anyone know the web site to do this calculation. (Maybe some
formula out there)
My result still a little bit different then the bank.
(because tax? or other fee charge by the bank ... )


=begin

  Calculate the payment for loan.
  This program try to find out the payment per year.
  It uses array of 2 elements to simulate symbolic algebra
calculation.
  [x,y] => x is real value, y is coefficient of payment per year.
  Note: no robust error checking for input value. GUI todo.

=end

#Re-define Array for our use. Note, it may-be better create our own
class.
class Array
  def *(int)
    self.map{|x| x*int}
  end

  def +(array)
    x = self.dup
    self.each_index{|i| x[i] += array[i]}
    x
  end

  def -(array)
    self + (array * -1)
  end
end


print "Capital = ? "
capital_in = gets.to_i
capital = [capital_in, 0] # to our format

print "Years = ? "
years = gets.to_i

print "interest rate = ? "
rate = gets.to_f

# calculate the sum of each year's interest
sum_int = [0,0]
(1..years).each {
  interest = capital * rate
  sum_int += interest
  capital -= ([0,1] - interest) # [0,1] means the payment for a year
}

# Now, find out what is exactly the payment per year (= X).
# We have formula like: total payment = capital + sum interest
# ==> years * payment/years = capital_in + sum_int
# ==> years * X = capital_in + [total interest, coef of X ]
# ==> years * X = capital_in + sum_int[0] + sum_int[1] * X
# ==> X = (capital_in + sum_int[0]) / (years - sum_int[1])

payment = (capital_in + sum_int[0]) / (years - sum_int[1])

# now, output in good format.
puts "-" * 30
puts "you loan from bank:"
puts "capital = #{capital_in}"
puts "years = #{years}"
puts "interest rate = #{rate}"
puts
puts "payment/year = #{payment}, payment/month = #{payment/12}"
puts "total payment = #{payment*years}"
puts "total bank gain = #{payment*years-capital_in}"

#=begin  More detail information for each year
  capital = capital_in
  puts "\nYear\tCapital\tInterest\tPayment"
  (1..years).each {|year|
    # puts "#{year}\t\#{capital}\t#{capital*rate}\t#{payment}" # <--
the #{capital} is not print correctly, why ??
    printf "%i\t%.2f\t%.2f\t%.2f\n", year, capital, (capital*rate),
payment
    capital -= (payment - capital*rate)
  }
#=end