input: Write a python function to plot a bar chart with given age and number of people data and save it as a PNG file.
  assert plot_bar_chart([18, 28, 38, 48, 58, 68], [500, 600, 400, 200, 100], 'test_plot.png') == 'test_plot.png'
  assert plot_bar_chart([20, 30, 40, 50, 60, 70], [300, 400, 500, 600, 700], 'test_plot2.png') == 'test_plot2.png'
output: import matplotlib.pyplot as plt
  
  def plot_bar_chart(ages, numbers, filename):
      plt.figure(figsize=(10, 6))
      plt.bar(ages, numbers, color='blue')
      plt.xlabel('Age')
      plt.ylabel('Number of People')
      plt.title('Age vs Number of People')
      plt.savefig(filename)
      plt.close()
      return filename