求xpath

<root>
  <row a ="1" b="asd"/>
            .
            .           
            .
  <row a ="5" b="adsfasd"/>
  <row a ="5" b="要这个"/>                  
            .
            .           
            .
  <row a ="sdfas" b ="23423sfas"/>
</root>

请问我想要知道第二个a="5"的row节点的b属性值,的xpath怎么写?
"root/row[@a=5 and ?????] /@b "