What will be the relative atomic mass of the element A, which naturally occurs in the environment, if scientists find 612 atoms of A-15, 829 atoms of A-16, and 244 atoms of A-17?

First, you have to get the percentages of all these isotopes of Element A. Add them all up together.
612+829+244 = 1685
Then, find the percentages of each - use the formula ([Number of Atoms/Total in the Sample] x 100)
([612/1685] x 100) = 36.32% of A-15.
([829/1685] x 100) = 49.20% of A-16
([244/1685] x 100) = 14.48% of A-17.
Now do what youve always done - multiply the percentages by the masses, add them all up, and divide by 100.
(36.32 x 15) + (49.20 x 16) + (14.48 x 17)
/100
= 15.78