How much I need to save for my children education

I am 40 years old having two sons, elder aged 7 and younger aged 3. I have invested Rs. 5 lacs in Mutual funds (Balanced funds) (Long term horizon) also have taken a policy of Rs. 5 Lacs (50k investment for 5 years, locking period) in HDFC young star plan for my younger son and LIC policy for my Elder Son. and I have a current savings of Rupees 5 Lacs. Would appreciate if you could let me know how much more investment I need to secure my boys education when they will be 20 years old?