why python print " num += 1 " invalid syntax? : <pre><code>balance = 5000
monthlyPaymentRate = 0.02
annualInterestRate = 0.18
num = 0
remaining_balance = 0.00

while num &lt;= 12:
      remaining_balance = (annualInterestRate/12 * (balance-(balance * monthlyPaymentRate))+ tem_balance
      num += 1

print(remaining_balance)
</code></pre>

<p>why python print " num += 1 " invalid syntax ?</p>
