Question 824197
This is a sequence not a series.  A sequence is an ordered list of numbers.  A series is a sum of a sequence.<P>
There is not a single answer for "what is the next number" in this sequence.  A sequence can have multiple rules.  As long as the rule makes sense (works) it is valid.<P>
The next number could be 54.  The rule that would make the number 54 is "add three to the first number and 4 to every number after the first number".<P>
The next number could be 55.  The rule that would make the number 55 is "add the number that equals the tens digit to each number."  39 has 3 as the tens digit, so the next number is 39+3 = 42.  All the numbers in the 40's have 4 as the tens digit, so add 4 to each one - 42+4 = 46, 46+4 = 50.<P>
50 has 5 in the tens digit.  50+5 = 55.<P>
There may be other valid rules that result in a different next number.