Think about the following list of number pairs. three is the first number of a pair and 8 is the second number.
3-8
4-11
5-14
6-17
10-29
if 50 is the first number what is the second number?
if 200 is the first number what is the second number?
if 89 is the second number what is the first?
if a number n is the first number, what is the second number?

A) if 50 is the first number what is the second? 

50*3 - 1 = 150 - 1 = 149 

B) if 200 is the first number, what is the second? 

200*3 - 1 = 600 - 1 = 599 

C) if 89 is the second number, what is the first? 

[first]*3 - 1 = 89 ==> [first]*3 = 89 + 1 ==> [first]*3 = 90 ==> [first] = 90:3 ==> [first] = 30. 
So, 30 is the first number 

D) if a number n is the first number, what is the second number? 
 3n-1 

Hope this helps ya out =)

D.