How to Calculate the Atomic Mass of Contaminated Iodine?

[Ki-nana18]

Homework Statement

Naturally occurring iodine has an atomic mass of 126.9045. A 12.3849-g sample of iodine is accidentally contaminated with 1.00070 g of ,¹²⁹I a synthetic radioisotope of iodine used in the treatment of certain diseases of the thyroid gland. The mass of¹²⁹I is 128.9050 . Find the apparent "atomic mass" of the contaminated iodine.

 

[Borek]

How do you calculate atomic mass knowing isotopic composition? Do you know what 'weighted average' is?