Answer:
192 is the missing number in the sequence.
Explanation:
Given sequence is 131, 517, ?, 123.
First write the numbers without comma as 131517?123.
Break it in two digit numbers as 13, 15, 17, _ _ 23
Complete the sequence with the missing two digit odd numbers.
13, 15, 17, 19, 21, 23
Now rewrite the sequence without comma again.
131517192123
Place the comma after every three digits.
131, 517, 192, 123
Therefore, the missing number is 192.