Let k = the constant amount added to the sequence since this is an arithmetic sequence
x = missing number
You have to formulate an equation based on the sequence:
6 + k = x or k = x – 6
x + k = 30
Substitute the value of k to the second formula:
x + x – 6 = 30
2x = 36
x = 18
Therefore, the missing number is 18.