Aflea weighs 8 decigrams, and an ant weighs 3 milligrams. how much more does a flea weigh than an ant?

In this question weight of a flea and an ant is given, but their units are different. So comparing and finding out the weight of flea and ant would be difficult. First thing that needs to be done is to bring the units of both the flea and the ant under same unit. 
Weight of flea = 8 decigrams
Weight of ant = 3 milligrams.
Now let us convert the weight of the flea from decigrams to milligrams.
We already know that
1 decigram = 100 milligram
Then
8 decigrams = 100 * 8 milligrams
                     = 800 milligrams
Then 
The weight by which the flea is greater than the ant = (800 - 3) milligrams
                                                                                   = 797 milligrams.
So the flea weighs 797 milligrams more than the ant.  

The flea weighs 500 milligrams more than the ant does.
Hope this helped!