If someone says Jazakum Allahu khairan to you:
You can reply: Wa antum fa jazakum Allahu khairan
Or: wa yaakum
fem: Jazaki Allahu khairan -wa yaaky
masc: Jazak Allahu khairan -wa yaak
If someone says Jazakum Allahu khairan to you:
You can reply: Wa antum fa jazakum Allahu khairan
Or: wa yaakum
fem: Jazaki Allahu khairan -wa yaaky
masc: Jazak Allahu khairan -wa yaak