325
Pattern: 1, 4, 15, 64, [325]
1 = 1 x 1
2 = 2 (incrementing by 1) x (previous 1 x 1 +1) = 2 x 2 = 4
4 = 3 (incrementing by 1) x (previous 2 x 2 +1) = 3 x 5 = 15
16 = 4 (incrementing by 1) x (previous 3 x 5 +1) = 4 x 16 = 64
X = 5 (incrementing by 1) x (previous 4 x 16 +1) = 5 x 65 = 325

X = 325