First, you change 6 3/8 and 3 1/4 to a common denminator. 8
6 3/8= 6 3/8
3 1/4= 3 2/8 (x2)
Then add those.
6 3/8+ 3 2/8 = 9 5/8
Then subtract 22 1/2 and 9 5/8. (Common denominator)
22 1/2= 22 4/8 (x4)
9 5/8= 9 5/8
the 3rd bag is 12 7/8 lbs.
You can also add each bag (lbs) to see if it gets to 22 1/2
Hope this helps!