Number System Practice Test 1

In the sequence 17, ..., 18, 15, 19, 14, 20, 13, the next alternate number is one more than the previous alternate number, i.e., 17 + 1 = 18, 18 + 1 = 19, 19 + 1 = 20. Also the second number and fourth number differ by 1, i.e., 13 + 1 = 14, 14 + 1 = 15, 15 + 1 = 16 is the required missing number).
[* means add the first number to the twice of the second number].