'How can I compare a number to a previous highlighted number?

I'm stuck with few problems in google sheets. I have a solution to get the last number in a column

=ARRAYFORMULA(lookup(2,1/(A1:A20<>""),A1:A20))

but I'm not able to figure out how to solve the below scenarios. Any help would be greatly appreciated!

If 7b is more than 3 points compared to the last highlighted (green) number in 7B, then show "true"
If 7b or 5a is close to the last highlighted (green) number in B (ex: between a range of 1-3), then show "alert"
If the last number entered was in 8a then enter c7 as "done"

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