Problem Solving (PS) | The tick marks on the number line shown are equally spaced. What is

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Because the tick marks on the number line for an AP, not a GP. 

Place a GP on the number line and see what you get: 1, 4, 16, 64...
Now place an AP  on the number line: 4, 8, 12, 16, 20...
Where do you get ticks placed at equal intervals? In an AP because that is how we define an AP - each term is at a fixed distance from the previous term. ­

Here are some blog posts on Sequences:

https://anaprep.com/algebra-introducing-sequences/
https://anaprep.com/algebra-a-difficult ... sequences/