Answer for 4/5/10 Review Problem 7.

Note that V, A, <, and > represent down, up, left, and right arrows,
respectively.

Computation for Q1:
Bar 2 contains Q1, the 25th percentile.  Here is the
interpolation diagram for bar 2:
            40 
  +----------------------+     x     5 
  |   5                  |    --- = ----
  | +---+                |     1     40
  | |   |                |
  V V   V                V    x = 0.125, so
 20   25                60    Q2 = 1 + x = 1.125.
  +---+------------------+    
  |   |                  |
  |   |                  |
  |   |                  |
  +---+------------------+
 1.0  x                 2.0
 A A  A                  A
 | |  |                  |
 | +--+                  |
 |  x                    |
 +-----------------------+
             1.0

Computation of Q3:
Bar 3 contains Q3, the 75th percentile.  Here is the
interpolation diagram for bar 3:
            30 
  +----------------------+     x      15 
  |     15               |    --- =  ----
  | +--------+           |    0.5     30
  | |        |           |
  V V        V           V    x = 0.25, so
 60.0        75         90    Q2 = 2.25.
  +----------+-----------+    
  |                      |
  |                      |
  |                      |
  +----------+-----------+
 2.0  x     2+x         2.5
 A A         A           A
 | |         |           |
 | +---------+           |
 |     x                 |
 +-----------------------+
            0.5

Finally, IQR = Q3 - Q1 = 2.25 - 1.125 = 1.125.