print sprintf( "%.30f",     66.5 * ( ( 100 + 19 ) / 100 ) ), "\n79.135
+000";
print sprintf( "%.2f",   66.5 * ( ( 100 + 19 ) / 100 ) ), " <-- wrong,
+ expected 79.14\n";
;
print sprintf( "%.30f",     66.5 * (   100 + 19 ) / 100   ), "\n79.135
+000";
print sprintf( "%.2f",   66.5 * (   100 + 19 ) / 100   ), " <-- ok = 7
+9.14\n";
[download]

produces:

79.134999999999990905052982270718
79.13500079.13 <-- wrong, expected 79.14
79.135000000000005115907697472721
79.13500079.14 <-- ok = 79.14
[download]

Which may make the difference more obvious.

update: deleted incorrect example about integer Vs floating point calculations.