Inactive Tutor answered • 04/30/15
Tutor
New to Wyzant
There is no solution to this problem. You have two gaps. The pattern suggest by the first four entries (including the first gap as the third entry) suggests a pattern in which each additional change is one less than the last change.
18 + 3 = 21
21 + 2 = 23 (the missing number)
23 + 1 = 24.
This pattern would make the fifth entry (your second gap) also 24
24 + 0 = 24
Then the sixth entry would be 23
24 + -1 = 23
But the sixth entry (30) shows this pattern to be errant.
What actually happened was your teacher was looking for a simple arithmetic progression with each number 3 more than the last and either he/she mistyped when he/she wrote down the problem or you did when you copied it.
It is possible that a much more complicated pattern is at play, but unlikely.
