The Baumol Model

Hi sir, I'm doing exercises in Opentuition Notes. The question is "A company forecasts a cash requirement of $1,500,000, the use being constant throughout the year. The company has investments in excess of this amount which are earning 9.5% p.a. The company earns interest of 5% on their current account bank balance. The cost of selling investment is 150 per transaction. EOQ is $100,000". I don't understand while the "Interest loss on investments is calculated by (100,000+1,500,000)/2 * 9.5%. Why do we have to add 100,000 here? The opportunity cost is for 1,500,000, I think so, because that's the whole demand. Please explain it to me, thank you.