[cydonia]

This is a atomic mass problem. Basically I have to fill in the blanks of 3 major isotopes for silicon.  The 3 being 28 Si 29 Si and 32 Si. One of the blanks requires me to find the mass(amu) of 29 Si, the problem gives the abundance for 2 of the isotopes and mass for 2 with exception of 29 Si, also no average mass was given. How can I find the mass of 29 Si?

[Borek]

Write formula for observed atomic mass of silicon (take from the periodic table) as a fucntion of isotopes abundance, solve for unknown abundance of 29.

[cydonia]

I got the abundance of the unknown as the two other abundance were 4.7 and 3.09 so wouldn't that make the unknown 92.21?