physics
posted by linda .
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?

The boat sinks when the weight of a hemisphere of displaced water equals the weight of the hemisphere ball AND the pennies inside.
Write that as an equation and solve for the mass of pennies.
Divide that by mass of a penny to get the number of pennies. The volume of the pennies doesn't matter, because is is the hemispherical ball that displaces the water. , 
I don't understand how to solve :(