The arithmetic sequence formula:
[tex]a_n=a_1+(n-1)d[/tex]
We have:
[tex]a_5=3.2,\ a_9=12.0[/tex]
Calculate d (common difference)
[tex]a_9-a_5=4d\\\\4d=12.0-3.2\\\\4d=8.8\ \ \ |:4\\\\d=2.2[/tex]
Calculate 31th term:
[tex]a_{31}=a_5+26d\\\\a_{31}=3.2+26\cdot2.2=60.4[/tex]
