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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 cm^{3}. 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.

Solution

Mail to Ken