Given
Average atomic mass =16.2u.
Let, % of 168X=a;% of188X=(100−a)
Average atomic mass is generally calculated using the formula,
A1×x100+A2×Y100
Where,
A1,A2⇒Atomic masses
X,Y⇒Respective percentages
using the given data,
16.2=16×a100+18×100−a100
1620=1800−2a
a=90%(168X)
100−a=10%(188X)