1)Naturally occuring boron is made up of 20% by mass of 10B isotope and 80% by mass of 11B isotope.Calculate the relative atomic mass of boron. (2)Natural Silicon containing ores contains 92% of 28Si, 5% of 29Si and 3% of 30Si.Calculate the relative atomic mass of Silicon.

Answer 1

(0.2*10) + (0.8*11) = mass B.
To be accurate you should have the masses of 10B and 11B given and plug those values in instead of 10 and 11.