Answer:
a) To add the weights, we can start by adding the pounds and then adding the ounces separately.
12 lb + 11 lb = 23 lb
15 oz + 6 oz = 21 oz
So, the total weight is 23 lb 21 oz.
b) To subtract the weights, we can start by subtracting the pounds and then subtracting the ounces separately.
18 lb - 13 lb = 5 lb
10 oz - 11 oz = -1 oz
Since we have a negative value for the ounces, we need to borrow 16 ounces from the pounds. So, the final result is:
4 lb 16 oz - 1 oz = 4 lb 15 oz.
Therefore, the result is 4 lb 15 oz.