On the 1st of January 2020, David turns 20 years old and he starts thinking of his retirement plan. He plans to make a saving at the end of each year. This saving is increasing 7.5% every year. His saving is in a mutual fund, which gives the promised return at the first 10 years 5% p.a., then reducing every five years by 0.5% until 2.5%. The return is subjected to personal tax income. The average personal tax income rate is 25%. At the age of 60, 1st of January 2060, David plans to get 10 million baht.

How much should he save at the end of 2020?