Answer :
The general formula for an arithmetic sequence is given by
[tex]a_n=a_1+d(n-1)[/tex]where
a_1=4
d=3
substitute
[tex]\begin{gathered} a_n=4+3(n-1) \\ a_n=4+3n-3 \\ a_n=3n+1 \end{gathered}[/tex]so
For n=20
substitute
[tex]\begin{gathered} a_{20}=3(20)+1 \\ a_{20}=61 \end{gathered}[/tex]