Question

Insert the missing number. 7, 26, 63, 124, 215, 342, (....)
A) 391
B) 421
C) 481
D) 511

Answer 1

(2³ - 1) = 8-1 = 7
(3³ - 1) = 27-1 = 26
(4³ - 1) = 64-1 = 63
(5³ - 1) = 125-1 = 124
(6³ - 1) = 216-1 = 215 
(7³ - 1) = 343-1 = 342
so the next number will be 8 (8³ - 1) = 512 - 1 = 511. 

Answer 2

Given:

The sequence of the numbers here is , $(5^3-1)=(125-1)=124$$(5^3-1)=(125-1)=124$

7, 26, 63, 124, 215, 342, etc.

Formula used:

$(x^3-1)$

Solution:

By applying the formula to the first value of the sequence,

$(2^3 -1)=(8-1)=7$

$(3^{3} -1)=(27-1)=26$

$(4^3-1)=(64-1)=63$

$(5^3-1)=(125-1)=124$

$(6^3-1)=(216-1)=215$

$(7^3-1)=(343-1)=342$

Hence,

$(8^3-1)=(512-1)=511$

Answer:

D) 511