High data usage on tablet wifi started June 11

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http://androidforums.com – I have studied many posts on this subject but still not got to the bottom of this problem .. After 6 months of use, my son's tablet suddenly started using large amounts of data (on 11th June 2013), and has continued to use 2GB+ every day since. He only uses wifi, but has managed to exceed its 60GB monthly limit. Before, the daily usage was about 500MB/day for the whole household. I immediately told him to cut back on youtube but that didn't make much difference. I also checked the download history for new apps around the 11th. A data monitoring app attributes the data to "system", (General)