Let's analyze the increments for each step of the sequence:
Each step we add 3 to the previous number.
Since we need the 20th, from what we saw, in the 20th term we will have added three 20 times.
So the procedure is: calculate how much is 3 times 20 and then add that to the first term of the sequence.
So we add 60 to the first term to find the 20th term:
Answer: 124
