The % is a reserved character in XML like <, >, etc. Use %% for each % you are using in the string resource.
%
<
>
%%
Encoding each as a unicode character in the xml works for me:
<string name="test">100\u0025 foo 40\u0025bar</string>
Using CDATA may work..
<item><![CDATA[100% foo 40%]]></item>