The lower quartile is the second smallest and the upper quartile is the fourth smallest (second largest).
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First arrange the data set in ascending order. Suppose the data set consists of n observations. the index for the lower quartile is (n + 1)/4 and the index for the upper quartile is 3*(n + 1)/4. Find the values that correspond to the number in these positions in the ordered list. For example, if n = 15, then lower index = 4 and upper index = 12. So the lower quartile is the fourth number and the upper quartile is the twelfth. If n is large, you may skip the +1 and just look at n/4 and 3n/4. Often the indices are not integers. Then, if you are a beginner (nd the fact that you asked this question suggests that you are), find the nearest whole numbers for the two indices. Otherwise you need to interpolate and that is a whole new ball game!
In statistics the class boundaries are numbers that separate classes without forming gaps. To find the lower, you subtract 0.5 and to find the upper, you add 0.5. If the class is 3-7 the lower class boundary would be 2.5 and the upper 7.5. The upper boundary of one class will equal the lower boundary of the next class.
You would first need to know the amounts of each quartile. Then use your formula to place the numbers inside.
Find the minimum and maximum values from the given data. Then range is the difference between maximum and minimum values.