4lb 12oz + 7lb 11oz =

4 lb 12 oz  + 7lb 11 oz

Adding the ounces we have 11 + 12 = 23 oz

And since there are 16 ounces in one pound, this equals 1 lb 7 oz

And adding back the rest, we have

4 lb + 7lb + 1 lb + 7 oz  =

12 lbs 7 oz