A hollow rubber ball with a diameter of 5.57 cm and a mass of 38.9 g is cut in half to make a boat (in a bathtub) for American pennies made after 1982. The mass and volume of an American penny made after 1982 are 2.5 g and 0.36 cm3. How many pennies can be placed in the boat without sinking it?

Answer 1

I explained how to do this yesterday.
See
http://www.jiskha.com/display.cgi?id=1319564782

The solution utilizes Archimedes' Principle.

[(pi*Dball^3)/12]*Pwater = (1/2)*38.9g + Mp

where Pwater = 1.00 g/cm^3 is density of water and Mp is the mass of copper pennies, in grams.

45.24 g = 19.45 g + Mp
Mp = 25.79 g

Mp is a bit more than the mass of ten pennies. An eleventh would sink it.

Answer 2

thank you drwls !