The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 168X and 188X in the sample?

Let the percentage of 168X be A.
Then the percentage of 188X is (100 - A).
The average atomic mass is given by:
Average atomic mass = (Percentage of 168X × Atomic mass of 168X) + (Percentage of 188X × Atomic mass of 188X)
16.2 = (A/100 × 16) + ((100 - A)/100 × 18)
1620 = 16A + 1800 - 18A
1620 = 1800 - 2A
2A = 1800 - 1620
2A = 180
A = 90
Therefore, the percentage of 168X is 90% and the percentage of 188X is 100 - 90 = 10%.