Donald accepted the following bills drawn by Andrew on 8th march, 2014. $4,000 for 4 months. On 16th march, 2014, $5,000 for 3 months. On 7th April, 2014, $6,000 for 5 months. On 17th May, 2014, $5,000 for 3 months. He wants to pay all the bills on a single day. Find out average due date.