First, put both into just ounces. (1lb = 16oz)
6lb 4oz = 100oz.
5lb 10oz = 90oz.
Second, subtract.
100 - 90 = 10oz.
This is the answer. Note that 1lb = 16lb, so we can't put 10oz into pounds.
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Hence, the answer is 10oz.
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Wolfyy :)