Can you find a pattern and find the missing number in the series given below - 84, 155, 258, ? , 584, 819.

Answer 1

The missing number is 399.

Explanation:
84 = 4^3 + 4^2 + 4^1
155 = 5^3 + 5^2 + 5^1
258 = 6^3 + 6^2 + 6^1
584 = 8^3 + 8^2 + 8^1
819 = 9^3 + 9^2 + 9^1

Thus,
399 = 7^3 + 7^2 + 7^1

Answer 2

Pattern: 84, 155, 258, 399, 584, 819
Missing number: 399

Logic:
Difference of Difference between two numbers is incremented by 6

Difference between 2 numbers :

155-84 = 71
258-155 =103
399-258 =141
584-399 = 185
819-584 = 235

And the difference of difference :

103-71 = 32
141-103 = 38
185-141 = 44
235-185 = 50

Difference incremented by 6