Where does the 3600 comes from in part a in question 2?

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The manager at O'Donalds has decided to move forward with construction for the new drive-up window. She is concerned that the ice machine may not be able to keep up with the increased volume of customers. The machine makes ice all night, so it is filled with 70 lbs of ice at the start of the day. The existing machine can produce 45 lbs/hour