The missing number is 10 - you must take the sum of the absolute differences (i.e. positive value) between the three top numbers in each box to produce the bottom numbers...
Box 1: [4-2]+[6-4]+[6-2]=8
Box 2: [7-1]+[7-2]+[2-1]=12
Box 3: [9-4]+[9-1]+[4-1]=16
Box 4: [6-2]+[6-1]+[2-1]=10
There were several correct answers without solutions. Well done to Lu & TF, who got both the right answer and the reasoning behind it.
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