11.8 to three significant numbers
Add the two values to find the average mass
In order to calculate an atomic mass you do the following:
Sum the products of mass of each isotope and its relative abundance (in percentage) and then divide that sum by 100.
(The percentages must always add up to 100.)
That procedure works for elements with more than just two isotopes, you simply keep adding to the sum part of the equation (see below for the equation for four isotopes). This works with mass spectra, which provides the mass of isotopes (the position of a peak on the x-axis) and their relative abundance (height of the peak). The equation can be used with any unit for the mass too, amu, or g.
Solution to this problem
In this case the equation becomes:
For four isotopes the equation is: