Answer
The abundance of $^{151}Eu$ is 48.013 %.
The abundance of $^{153}Eu$ is 51.986 %.
Work Step by Step
We may relate the two abundances by $x$, the abundance of $^{151}Eu$ and $(1-x)$, the abundance of $^{153}Eu$. Therefore, we may write the following equation calculating the average atomic mass:
$151.96=150.9196*x+152.9209(1-x)$
$151.96=-2.0013x+152.9209$
$x=\frac{152.9209-151.96}{2.0013}$
$x=0.48013$
$(1-x)=0.51986$
