In this case, we'll have to carry out several steps to find the solution. 
Step 01:
Data
15 16 28 47 40 51 52 57 68 
5 number summary = ? 
Step 02:
5 number summary: 
 15 16 28 47 40 51 52 57 68 
min 1st median 3rd max 
minumum value = 15 
1st quartile = (16 + 28) / 2 = 22 
median = 40 
3rd quartile = (52 + 57) / 2 = 54.5 
maximum value = 68 
That is the total solution.