Since 3.2 is not a whole number (buckets cannot be split into small portions), and each bucket can only hold 30 golf balls, 4 buckets are needed to hold all golf balls.
Or else, if there is only 3 buckets, only 30x3 = 90 balls can be held, meaning there are 6 left.
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http://www.coolmath.com/algebra/08-lines/11-finding-equation-line-point-slope-01