Time Value of Money Problem

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Q. Mr. and Mrs. Wong have two children age 5 and 7. They want to start saving for their children's education. Each child will spend 6 years at college and will begin at age 18. College currently costs $20,000 per year and is expected to increase at 6% per year. Assuming they can earn an annual compound return of 12% and inflation is 4%, how much must they deposit at the end of each year to pay for their children's educational requirements until the youngest is out of school? Assume that educational expenses are withdrawn at the beginning of each year and that the last deposit will be made at the beginning of the last year of the youngest child.

Difficulties :
1. 2 inflations, 1 for education, another one for economical inflation. How to differentiate this two?
2. Should we take the mean to the two inflation?
3. Overlapping of school times for the two children in between, how to trade off PV?