To figure out how many boxes of diapers you will need, you should:
Decide on the time period you are buying for.
Check the number of diapers a baby in that stage of development is likely to use.
Check the number of diapers per box.
Multiply the daily number of diapers for that stage by the number of days in that period.
Divide the answer in Step 4 by the number of diapers in the desired box.
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