Manage single items with different MRP


Can anyone tell me how I can manage an item with two different MRP? Suppose

First I buy… Item X - Qty 2 - Price 20.00 and make a stock entry.
Next time the supplier send the item with increased mrp
Next, I buy… Item X - Qty 5 - Price 30.00

how will i make the stock entry

Also, how will these show on POS if I have to sell 4 pieces?