The astronaut then measures the abundance of silicon on the new planet, obtaining the following results: isotope abundance (%) mass (amu) 28si 73.71 27.98 29si 14.93 28.98 30si 11.36 29.97 what is the atomic mass of silicon for this planet?

To compute for the average atomic mass of silicon, we need to have the atomic masses of each of the isotopes and their relative percent abundance. We then get the summation of the product of the atomic mass and percentage. 

atomic mass = (.7371) 27.98 + (0.1493) 28.98 + (0.1136) 29.97 = 28.36 amu

The answer to this problem is. 67