There are 16 oz in a pound.
Add the ounces
15+8 is 23 oz
When converting to a pound you can simply divide 23/16 or subtract in this case since it is a small number
23-16 is 7
in 23 oz it is equal to 1lb and 7 oz
Finally add the original 4, the 6 and the1lb and 7oz making it
11lbs,7oz