Answer:
Mason counted 24 more birds at his feeder than Rosa counted at hers
Step-by-step explanation:
Average = Total or sum/number of items
Therefore, Total or sum = Average × number of items
For Mason,
An average of 18 birds in 20 day is equivalent to a total of
Total birds = 18 × 20
         = 360 birds
For Rosa,
An average of 21 birds each day for 21 days is equivalent to a total of
Total birds = 21 × 16
          = 336 birds
Difference between the total number of birds for Mason and Rosa
= 360 - 336
= 24 birds
