55

Number
$55$ (fifty-five) is:
 * $5 \times 11$


 * The $10$th triangular number after $1, 3, 6, 10, 15, 21, 28, 36, 45$:
 * $55 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = \dfrac {10 \times \left({10 + 1}\right)} 2$


 * The $10$th Fibonacci number, after $1, 1, 2, 3, 5, 8, 13, 21, 34$:
 * $55 = 21 + 34$