Most elements have several stables but some such as lithium, boron, and copper have 2. consider one such element (it doesn't matter which one). if the abundance of one isotope is 0.1266 what is the abundance of the other isotope?

See below.

Step-by-step explanation:

OK, let's consider lithium. It consists of 7.59 % ⁶Li and 92.41 % ⁷Li and has an atomic mass of 6.94 u.

The atomic mass of ⁶Li is 6.015 u, and that of ⁷Li is 7.016 u.

What would the atomic mass of lithium be if it consisted of 87.34 % ⁶Li and 12.66 % ⁷Li?

The atomic mass of X is the weighted average of the atomic masses of its isotopes.

We multiply the atomic mass of each isotope by a number representing its relative importance (e.g., its fraction of the total).

Set up a table for easy calculation

0.8734 × 6.015 u = 5.254   u

0.1266 × 7.016 u = 0.8882 u

              TOTAL =  6.142    u

The atomic mass of lithium would be 6.14 u instead of 6.94 u.