Concept:/Formula: Divisibility law of 11: If the difference of the alternating sum of digits of the number is a multiple of 11 (e.g. 2343 is divisible by 11 because 2 - 3 + 4 - 3 = 0,which is a multiple of 11). Calculation: If the number 6X6 is divisible by 11, then ⇒ 6–X+6=11 ⇒ 12–X=11 ⇒ X=12–11 ⇒ X=1 ∴X=1