Agrain of sand is made up of 1.38 × 10-3 grams of oxygen and 0.00122 grams of silicon. what is the total weight, in grams, of the grain of sand?

               2.6 × 10⁻³ g   (Or)    0.0026 g

                 Mass of Oxygen  =  1.38 × 10⁻³ g

                 Mass of Silicon  =  0.00122 g  =  1.22 × 10⁻³ g

The total weight of grain of sand is calculated by adding the masses of Oxygen and Silicon as,

                        Total Weight  =  Mass of Oxygen + Mass of Silicon

Putting values,

                            Total Weight  =  1.38 × 10⁻³ g + 1.22 × 10⁻³ g

                            Total weight  =  2.6 × 10⁻³ g

Or,

                            Total weight  =  0.0026 g

i'm pretty sure its b i might be wrong though!