Families saving for college continue to invest in 529 plans. Total assets in these tax-advantaged investment plans grew by 9.1 percent to $247.9 billion in 2014, according to a report by the College Savings Plans Network.

More people opening up accounts as well as investment gains contributed to the growth in 529 assets last year. The total number of 529 accounts increased 4.1 percent last year to 12.1 million (see graphic below). The average 529 account size grew to $20,474, a 4.5 percent gain from 2013. But that's still barely enough to cover one year of school at a public four-year college.

During the 2013-14 academic year, the average cost of tuition, room and board at a public four-year college was $18,391,a jump of 1.2 percent. And at private nonprofit four-year colleges, the average cost was more than double that: $40,917—an increase of 1.7 percent from the previous year, according to The College Board.

And the gap between college savings and costs may grow if market gains slow: The College Savings Plans Network notes that 47 percent of all 529 accounts got no new contributions last year.

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