The Common Difference

A sequence that adds the same number every time is an arithmetic sequence, and that number is its common difference, d. In 4, 10, 16, 22 every gap is 6, so $d = 10-4$, which is 6.

Always subtract in the same direction: a term minus the term before it. In 30, 25, 20, that gives $d = 25-30$, which is -5. The sign carries meaning: a positive d means the sequence grows, a negative d means it shrinks.

Not every sequence is arithmetic. In 1, 3, 9, 27 each term is 3 times the one before, so the gaps are 2, 6 and 18, not equal. Before you name a common difference, check that it really is common to every gap.