Acertain element consists of two stable isotopes. the first has a mass of 68.9 amu and a percent natural abundance of 60.4 %. the second has a mass of 70.9 amu and a percent natural abundance of 39.6 %. what is the atomic weight of the element?

The average atomic weight of this element can be calculated as follows:
average atomic weight =
(atomic weight of first isotope)(its percentage of abundance) + 
(atomic weight of second isotope)(its percentage of abundance)

atomic weight of first isotope = 68.9 amu
its percentage of abundance = 0.604
atomic weight of second isotope = 70.9 amu
its percentage of abundance = 0.396

So, just substitute in the above equation to get the atomic weight of the element as follows:
atomic weight = (68.9)(0.604) + (70.9)(0.396) = 69.692 amu

Hope the image of the frequency formula