Martin earned the following scores on his last five tests.

98, 78, 84, 75, 91

what is the interquartile range of his scores?

98, 78 , 84, 75, 91

The interquartile range formula is the first quartile subtracted from the third quartile:

IQR = Q3 – Q1.
Step 1: Put the numbers in order.
75, 78, 84, 91, 98

Step 2: Find the median.
75, 78, 84, 91, 98

Step 3: Place parentheses around the numbers above and below the median. 
Not necessary statistically, but it makes Q1 and Q3 easier to spot.
(75, 78), 84, (91, 98)

Step 4: Find Q1 and Q3
Think of Q1 as a median in the lower half of the data and think of Q3 as a median for the upper half of data.
(75, 78), 84, (91, 98)

*(75, 78) ---> (75 + 78) ÷ 2 = 76.5
*(91, 98) ---> (91 + 98) ÷ 2 = 94.5

Step 5: Subtract Q1 from Q3 to find the interquartile range.
94.5 – 76.5 = 18

IQR = 18

The third box is a function