averaging a stock

Hi Experts, I am new to trading (1 year) and I think I might have done something silly but cant find clarification anywhere. I bought 100 shares at 143, then I bought another 100 at 102, then I bought another 43 shares at 169. After buying the 3rd lot, my average has increased to 131...

As i am a beginner pls. help me on averaging a stock. I buy 100 shares @ Rs. 200, now the share price go downwards and it comes to Rs. 150, now pls tell me how much share I should buy so that my average price comes to neat Rs.150 per share.