\begin{problem*}{30.64}
Two coplanar and concentric circular loops of wire carry currents of
$I_1=5.00\U{A}$ and $I_2=3.00\U{A}$ in opposite directions as in
-Figure P30.64. If $r_1=12.0\U{cm}$ and $r_2=9.00\U{cm}$, what
+Figure~P30.64. If $r_1=12.0\U{cm}$ and $r_2=9.00\U{cm}$, what
are \Part{a} the magnitude and \Part{b} the direction of the magnetic
field at the center of the two loops? \Part{c} Let $r_1$ remain fixed
at $12.0\U{cm}$ and let $r_2$ be variable. Determine the value of