11 Dhu al-Hijjah: 3 stations × 7 = 21 pebbles.
12 Dhu al-Hijjah: 3 stations × 7 = 21 pebbles.
Together: 21 + 21 = 42 pebbles.
Including the seven thrown on 10 Dhu al-Hijjah at Aqabah produces 7 + 42 = 49 throws across those three dates. The totals 42 and 49 simply refer to different scopes of counting.
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Book of Hajj: Return to Mina, Stoning the Three Jamarahs, and Days of Tashriq
11 Dhu al-Hijjah: 3 stations × 7 = 21 pebbles.
12 Dhu al-Hijjah: 3 stations × 7 = 21 pebbles.
Together: 21 + 21 = 42 pebbles.
Including the seven thrown on 10 Dhu al-Hijjah at Aqabah produces 7 + 42 = 49 throws across those three dates. The totals 42 and 49 simply refer to different scopes of counting.