The first three terms of an arithmetic sequence are 1, 10 and 19, respectively. What is the value of the 21st term?

1,10,19,... +9 each time

$T_n=dn+c$ where d is a common difference (9) and c is the first term - d

$T_n=9n-8$

Thus the 21th term is 

$T_{21}=9(21)-8=181$