First find how long it takes Janet to cross the lake: 60 miles / 30 miles/hour = <<60/30=2>>2 hours
Then find how long it takes her sister to cross the lake: 60 miles / 12 miles/hour = <<60/12=5>>5 hours
Then subtract Janet's travel time from her sister's travel time to find the difference: 5 hours - 2 hours = <<5-2=3>>3 hours
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