"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)
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? 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.
