Stainless steel is an alloy of iron containing 5 percent nickel and 0.175 percent chromium. how many atoms of nickel are present in a 84 g sample of this alloy?
answer in units of atoms

First you need to work out what mass of 84g of stainless steel is nickel, and because there's 5% you can simply divide by 20, giving 4.2g

For this you can use the equation 

The mass, as was worked out previously, is 4.2g
The Mr, which is found on the periodic table, is 28

Moles =  = 0.15mol

1mol = 6.023 x  atoms

So, 0.15mol = 0.15 x 6.023 x  = 9.0345 x  atoms