Question

At two weeks old, Tabby the Cat weighed 10 ounces. Now that he's older, Tabby weighs 15 pounds and 3 ounces. How much more does he now weigh?

Answer 1

"How much more than" hints that you need to subtract the original weight (10 oz.) from the current weight (15 pounds, 3 oz.).

Important:  16 oz. = 1 lb.

I'd suggest that you convert 15 lb, 3 oz. to oz, so that you can easily compare the cat's mature weight to its weight at 2 weeks.
  
    15 lb 3 oz
 -          10 oz    (subtract 10 oz)         
-------------------- 
            ?              This is "how much more he weighs now."

Example:  convert 2 lb 11 oz    to    oz.

Solution:     (2 lb) (16 oz) / (1 lb) + 11 oz.  Note how "lb" will disappear.

Answer 2

He weighs 223 more ounces now or 14 pounds and 3 ounces more