Question

An element has two isotopes of mass numbers 85 and 87 and its atomic mass is 85.4.

Answer 1

I think you are asking for the % composition of the two isotopes.

Let x be the % of mass number 85

100-x us the % of mass number 87

100 * 85.4 = 85x + 87 (100-x)

8540 = 85x + 8700 - 87x

2x = 8700-8540

x = 160/2 = 80

80% of mass number 85 and 20 % of mass number 87 are the composition.