This is an arithmetic sequence because each term is 1/8 more than the previous term. This is called the common difference and it is what defines an arithmetic sequence.
a(n)=a+d(n-1), where a is 1st term, d is common difference, and n it term #
In this case:
a(n)=3.25+0.125(n-1) or more neatly...
a(n)=3.125+0.125n
a(5)=3.75
