After graduating from JCU, David has been working in a local company as a finance manager for 2 years. He is currently earning $4,000 per month. Over the last 2 years, he has successfully saved $20,000 in his bank account. However, David is considering investing his savings to fund for his childrens education expenses. He feels that based on the increasing cost of education in Singapore, he will need $200,000 in 6 years to pay off his children overall education fees throughout their elementary school period. Assuming Davids salary stays constant throughout 6 years from now.

a. Currently David contributes 20% of his salary to his CPF, spends $1,000 on the necessities per month, and saves $500 for emergency cash monthly. To fill up the gap for his children education fund needed in part (a), how much more he should invest his disposal income (after expenses and emergency fund) every month to accumulate the additional fund he needs over the next 6 years? Assumes he can earn an average return of 12% p.a. on his investment?