Sinking a Cup With Pennies

My daughter is studying mass, volume and density this week.  I created this lab experiment for her class.  A Styrofoam cup has a mass of 3.5g and a volume of 60 cm3.  If you hold the cup under water and release it, it will float to the surface.  What is the minimum number of U.S. pennies needed to put into the cup to keep the cup from floating?

Extension: The class actually has to determine the mass and volume of the cup.  How would you find the volume of Styrofoam in a Styrofoam cup?

Source: Original.

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