To find the 20th term in this sequence, we can simply keep on adding the common difference all the way until we get up to the 20th term.
The common difference is the number that we are adding or subtracting to reach the next term in the sequence.
Notice that the difference between 15 and 12 is 3.
In other words, 12 + 3 = 15.
That 3 that we are adding is our common difference.
So we know that our first term is 12.
Now we can continue the sequence.
12 ⇒ 1st term
15 ⇒ 2nd term
18 ⇒ 3rd term
21 ⇒ 4th term
24 ⇒ 5th term
27 ⇒ 6th term
30 ⇒ 7th term
33 ⇒ 8th term
36 ⇒ 9th term
39 ⇒ 10th term
42 ⇒ 11th term
45 ⇒ 12th term
48 ⇒ 13th term
51 ⇒ 14th term
54 ⇒ 15th term
57 ⇒ 16th term
60 ⇒ 17th term
63 ⇒ 18th term
66 ⇒ 19th term
69 ⇒ 20th term
This means that the 20th term of this arithemtic sequence is 69.