Calculating the nth term of an Arithmetic Progression

Given information from the problem:

$a_1=10$

$a_6=40$

We wish to find $a_{20}$

$a_6=a_1 + 5d$

$40=10 + 5d$

$30=5d$

$d=6$

Now compute for $a_{20}$

$a_{20}=a_1 + 19d$

$a_{20}=10 + 19(6) = 124$