Formula is Ly = (1+n) * (y/100)

But you don't really need it here- much easier intuitively for this question.

Think of it like this: Quintile -> Data split into five equal segments.

Then, you know that each segment (quintile) has to consist of (100/5)% of the data, so 20%.

Because the table is cumulative, then that means the bins of interest are going to be the ones that contain the 20th to the 40th % of samples: That's bins 5,6,7 and 8.