There is a formula for the general term of an arithmetic sequence, and is given by
a_n = a_1 + (n - 1) x d
The question gives us the following information:
a_1 = 1
d = 10
Just plugging the information into the formula, you get the general term like this
a_n = 1 + (n - 1) x 10
a_n = 10n - 9
Now that we have the general term, we just substitute n = 20 to get our 20th coefficient.
a_20 = 10 x 20 - 9 = 191
Then you get both answers!
The general term is
a_n = 10n - 9
and the a_20 is
a_20 = 191