Question 816779
  <pre><font face = "Tohoma" size = 3 color = "indigo"><b> 
Hi,
Sequence 31, 51, 71, 91 ...
Arithmetic Sequence d = 20
{{{a[n]=a[1]+(n-1)*d}}}
{{{a[n]=  31 +(n-1)*20}}} ***

CHECKING ***
{{{a[4]=  31 +(3)*20 = 91}}}