Answer:
5.5 units
Explanation:
Period  Actual   Forecast  Absolute deviation
  1       16        20           4
 2       28        16           12 Â
 3       20        20           0
 4       24        30           6
Total                          22
Mean absolute deviation(MAD) = Sum of absolute deviation / Number of periods
Mean absolute deviation(MAD) = 22 / 4
Mean absolute deviation(MAD) = 5.5 units