Answer
$a_{21}=110$
Work Step by Step
We have to find $21$st term of the arithmetic sequence:
$10, 15, 20, 25, . . .$
For the general $a_n$ term we have
$a_n=a_1+d(n-1)$.
In our case
$d=a_2-a_1=15-10=5$
So
$a_{21}=a_1+d(21-1)=10+5(21-1)=110$
