1) Replace each range of values in column 1 with the middle value; for example, replace 5 <= t < 10 with 7.5
2) After you have done this, create a third column that is found by multiplying the value in the time column with its
corresponding value in the frequency column; for example, for the first row, the value in the third column is 15.0.
3) Sum this third column and divide this value by the sum of the frequency column. This is your answer to part a.
4) To find the answer to part b: add up the number of persons who have finished in fewer than 20 minutes and
divide this number by the sum of the frequency column.