Answer:
Between the second and third vertical lines, as shown in attached image.
Explanation:
To find the point between the 12 and the 20, subtract 12 from 20 and divide the result by two:
Then, add the result (4) to 12. The midpoint between the 12 and the 20 on this number line is 16. This makes sense, because 16 + 4 = 20.
Now you can see that the midpoint between 16 and 20 is 18. This point is not yet marked with a line on the number line, but that's where you put the dot.