from 200mg of carbondioxide, 10^21 molecules are removed. How many moles of CO2 are left?

things you'll need to solve the problem:
1) #molecules in a mol (Avagdro's number)
2) molecular weight of CO2 (units are g/mol)
3) #moles CO2 in 200 mg (200mg = 0.200 g)
4) #molecules CO2 in the #moles calcualted for 3)
voilà