Ok for example a sequence is 1 2 3 it is a set of numbers in a corresponding order. So to find a missing number in 2 4 _ 6 8 10, you would look at the numbers that you have, for example 6 8 10, each of them are two apart so is 2 and 4 so the answer would 6! But there are more difficult sequences such as 1 3 5 9 11 13 15 _
You could look at this one and think easy, its 17, but there is a 5 and a 9, those are 4 apart. So every three there numbers there is a 4 number difference while in the others there is a two. So you would say the missing answer is 19!Hope it helps.