What mass of magnesium would combine with exactly 16.0 grams of oxygen

1.65g MgO = 1g Mg

1.65 - 1 = 0.65 g of O in MgO

solve it using proportion:

1g Mg / 0.65g O = x (g) Mg / 16g O

or 1 / 0.65 = x / 16

24.6 g is the answer.

if 1 gram of oxygen requires 1.65 grams of Mg

then 16 grams of oxygen will require 16 (1.65) or 26.4 grams.