The mass of a carbon-12 atom is 12 amu. What percent of the mass is located in the nucleus?
I can only give a simple answer, so if others have better ones, great, but here's my shot:

An electron's mass is a lot less than a proton/neutron. 
"When talking about relative masses, we use the term atomic mass unit (amu). Using this unit, a proton has a mass of 1.007 amu, a neutron a mass of 1.008 amu, and an electron a mass of 5.45 X 10-4 amu."    

Basically, the proton and neutron is close to 1, and the electron is close to 0.

Carbon-12 means it's got a total of 12 protons and neutrons, and since carbon has 6 protons, that makes 6 neutrons. Since it's a normally charged carbon atom too, that makes 6 electrons. 

Can you do the calculations yourself with that? Because now you know the masses.

Also this value should be really close to 100% because of how electrons are so much lighter than protons and neutrons; protons and neutrons are located in the nucleus and the rest of the atom is mostly empty space + some electrons, so that's why the bulk of the mass is there.