Chromebooks have come from nowhere to grab nearly a fifth of U.S. school purchases of mobile computers, posing problems for Microsoft and possibly even Apple. The inexpensive laptops, which run Google software but are mostly sold by other companies, accounted for 19% of the K-12 market for mobile computers in the U.S. in 2013, according to a preliminary estimate by Futuresource Consulting. In 2012, Chromebooks represented less than 1% of the market, according to the research firm, whose estimate includes both tablets and notebook PCs but excludes desktop computers. Mobile computers running Microsoft Windows slid from 47.5% of that market in 2012 to 28% in last year’s third quarter, said Futuresource, which isn’t yet providing comparisons for all of 2013. Data from IDC analyst Rajani Singh that includes purchases by U.S. colleges and universities as well as K-12 schools shows a similar pattern.