The average cost price of item 'X' and item 'Y' is Rs.3000, the average cost price of item 'Y' and item 'Z' is Rs. 3500 and the average cost price price of item 'X' and item 'Z' is Rs. 3700. Items 'X', 'Y' and 'Z' are sold at the profit of 10%, profit of P% and loss of 40% respectively and the overall loss incurred in this process is Rs. 660 then find the value of P?