what to say if someone says jazakallah khair?
The common response to Jazāk Allāhu Khayran is wa ʾiyyāk (وَإِيَّاكَ‎), or wa ʾiyyākum (وَإِيَّاكُمْ‎) for plural, which means "and to you". A more formal reply is "wa ʾantum fa-jazākumu-llāhu khayran" (وَأَنْتُمْ فَجَزَاكُمُ ٱللَّٰهُ خَيْرًا‎) which means "And you too, may God reward you with goodness".