Largest mass of rock that wont sink the boat

[jason_r]

Homework Statement

You begin by measuring one of the hemispheres, finding that it has a mass of 21 g and a diameter of 8.6 cm

Basically you have a hemisphere shaped boat with mass 21g and diamter 8.6cm you have to find the heaviest mass of rock that u can put in the boat that won't sink it.

What is the mass of the heaviest rock that, in perfectly still water, won't sink the plastic boat?

Homework Equations

I equated the densities of the boat and water and solved for m

The Attempt at a Solution

I got 3.85kg as an answer

 

[Rake-MC]

I assume that this answer is wrong, that's why you're posting here. Am I right?
It would help if you posted your method to acquire your answer, as well as all the relevant values ie. densities.

 

[reit2094]

I'm on a similar problem. The textbook appears to say that I should try to set the average density of the sphere equal to the density of the water. So, I tried:
rou(water)=rou(ave)
rou(water)=1000kg/m^3
rou(ave)=[mass(sphere)+mass(rock)+mass(air in sphere)]/volume
Then I set the two equal to each other and solved for the mass of the rock. I've gone through it several times and can't come up with the correct answer.