Chem 221, struggling with this equation

Question

An unknown element is a mixture of isotopes ¹⁵¹X (atomic mass = 150.99 amu) and ¹⁵³X (atomic mass = 153.03 amu). The average atomic mass of X is 151.69 amu. What is the percent abundance of ¹⁵³X?

Answer 1

The average atomic mass is a "weighted" average based on percentages.

Let x = fraction that is X-151

Then 1- x = fraction that is X-153

(x)(150.99) + (1-x)(153.03) = 151.69 amu

150.99x + 153.03 - 153.03x = 151.69

2.04x = 1.34

x = 0.6569 = 65.69% is X-151

1- 0.6569 = 0.3431 = 34.31 % is X-153