How interest will be calculated

nikhil ((CA Final) Article) (363 Points)

01 June 2011  

alpha manufacturing p ltd is to supply good on credit of two month to the partnership firm.The partnership is to discharge the due to alpha manufacturing p ltd along with interest at 12% per annum regularly on due dates.

purchases150 lakhs of which30 were in the first quarter;90 lakhs were in the next six month; the balance30 lakh were in the last quarter. The purchase are evenly spread though the respective periods.??

answer is 2.80 interest

how this interest will be calculated ??plz help me to know this