I need help with this exercise. I have the answer but I do not know how to get to that point. Could anybody explain how to solve this problem?

Thanks in advance

We have a random sample of 20 observations. We need to find the lower and upper limits for the ratio of the sample variance divided by the population variance.
The limits are such that the probability that the ratio is below/above the limit is 0,025 (thus 95% will be within the limits).