You have the following numbers:
5.6
13.5
18.7
27
34
In order to find the interquartile range for the previous data you first find the median of the data.
The median, in this case is 18.7 (this is the second quartil Q2)
Next, you find the median of the first half of the data and aslo of the second one:
For the first half, the median is: 13.5 (this is the first quartil Q1)
For the second half you obtain: 27 (this is the third quartil Q3)
Quartils Q1, Q2 and Q3 determines the interquartil range.