This is the output from the second printf statement:

printf("The value of average is %f",average);

The information in the Format String (the part
enclosed in double quotation marks) is literally printed
out as output.  Where the %f Format Character is
situated within the Format String, the value of a specific
float is output.  The argument AFTER the Format String 
shows what specific value this will be: the value of the 
variable average.

In this example, since the value of average is 2.5,
the output is:

The value of average is 2.500000

When the %f Format Character is used, 6 decimal places 
will be given automatically.  To alter that, one can modify 
the Format Character:

%.2f will give 2 decimal places.
%.3f will give 3 decimal places.
%.4f will give 4 decimal places.
etc.