grep cartons [ab]-* |
{ while read x x x x x eggs x cartons x; do
    (( total_eggs+=eggs ))
    (( total_cartons+=cartons ))
  done
  echo "A total number of $total_eggs eggs in $total_cartons cartons makes an average of $(( total_eggs/total_cartons )) eggs per carton"
}

$> grep cartons [ab]-*
a-10.txt:number of eggs used is 55 in 4 cartons
a-4.txt:number of eggs used is 97 in 3 cartons
a-6.txt:number of eggs used is 22 in 1 cartons
a-8.txt:number of eggs used is 44 in 9 cartons
b-10.txt:number of eggs used is 87 in 5 cartons
b-4.txt:number of eggs used is 85 in 5 cartons
b-6.txt:number of eggs used is 36 in 6 cartons
b-8.txt:number of eggs used is 88 in 2 cartons

A total number of 514 eggs in 35 cartons makes an average of 14 eggs per carton