1. Interpolation

How would I use interpolation to find an estimate of the median and quartiles of a frequency distribution?

I've always struggled with statistics.

2. I'm sort of alright with the median, it's the quartiles which I get wrong.

3. The first quartile is where 25% of the data is less than or equal to the number and the third quartile is where 75% of the data is less than or equal to the number. I'm not sure what you mean by "interpolation". If you are saying that, say, the 25% point lies between two data points, taking halfway between is perfectly valid.

4. Interpolation - just a weird word?

I copied that word from an example question. Essentially, the question gave a distribution such as the one below, and asked me to 'using interpolation, estimate the values of the quartiles' I know that the lower and upper quartile both have seperate formulae, I just can't remember them and they don't seem to be displayed upon the internet.

Interval Frequency
00<x<05 6
06<x<10 10
11<x<20 12
21<x<35 4
35<x<40 2

Which is an example I've made up off of the top of my head, how would I estimate the quartiles without drawing a cumulative frequency graph or the like?