Well, drop 65 cents (the first 250 grams) to get 90 remaining cents. These are 900 additional grams. So, the mass is 0.25+0.9=1.15 kg.
The weight of a package for which the charge is $1.55 is 1150 grams. This total includes an initial charge for the first 250 grams, plus additional charges for extra weight. The calculation involves subtracting the initial charge and determining how many increments of 100 grams can be covered by the remaining charge.
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