1-20, 21-40, 41-60, 61-80, 81-100, 101-120
It should be 81, not 91
Next, class middle value is the median
I hope you know what frequency is. Frequency is the number of items from you data set that fall in the intervals that you have stated in the table. So, for the interval "1-20" your frequency is 2 because there are 2 items in the data that fall in the interval "1-20". Likewise, for 21-40, your frequency is 3, and so on.. when you get the frequency of all the ranges you have stated in the table, the sum of the frequencies should add up to the number of data you have in the set, which is 27.
Cumulative frequency(C.F) is the frequency of a random variable below a particular level. It tells how often the value of the random variable is less than or equal to a particular reference value.
So for 1-20, your C.F. is 2
for 21-40, the C.F. is 2+3= 5
for 41-60, the C.F is 2+3+6= 11
for 61-80 the C.F. is 2+3+6+7 = 18
for 81-100 the cf is 2+3+6+7+7 = 25
for 101-120 the cf is 2+3+6+7+7+2 = 27