Pretty basic S1 Histogram question

We just started Histograms in class the other day, and we did a question in class. I have a question about it though.

I am talking about part C. I know the answer, but I don't know why?

When you find the number of students who took 80 seconds or less (using density, k, etc), how do you know that you don't need to include the bar 80-85? How do you know you only take the readings from the bars to the left of where it says 80(so the 40-60 bar, the 60-70 bar and the 70-80 bar)?

80 appears in two bars, because the bars overlap, so how do you know which bar to use when 80 appears twice and there are two bars that overlap which have 80 in them?

Is there some sort of rule for continuous data, something that perhaps involves two greater/less than (or equal to) signs, with one of those being an or equal to sign? Otherwise, how do you know what to do when you want the result of the bars under 80 seconds or less, where two bars have 80 in them?

I know how to do it, but I don't know why you don't include the bar 80-85, which has 80 in.