91

Number
$91$ (ninety-one) is:


 * $7 \times 13$


 * The $13$th triangular number after $1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78$:
 * $91 = \displaystyle \sum_{k \mathop = 1}^{13} k = \dfrac {13 \times \left({13 + 1}\right)} 2$