Convert to the same unit and give the total weight-
20kilograms+700grams+0.004kilograms+300milligrams+500centigrams+0.008decagrams+10,000,000micrograms
equals

Hi ciao-ga!!
I will calculate the total weight in grams, to do that I must convert
all the terms to grams.
1kg = 1,000g, then 
20kg = 20 x 1000g = 20,000g

700g = 700g, no problem here.

0.004kg = 0.004 x 1,000g = 4g

1mg = 0.001g, it is the thousandth part of a gram, then 
300mg = 300 x 0.001g = 0.300g

1cg = 0.01g, it is the hundredth part of a gram, then 
500cg = 500 x 0.01g = 5g

1dg = 10g, then 
0.008dg = 0.008 x 10g = 0.08g

1microgram = 0.000001g, it is the millionth part of a gram, then 
10,000,000micrograms = 10,000,000 x 0.000001g = 10g

Now we can do the sum, because we just have all terms in the same unit
(grams).

Total Weight = 20,000g + 700g + 4g + 0.300g + 5g + 0.08g + 10g =
20,719.380g

Now you can convert this result to the units that you wish using the
conversion factors.

Note that the knowledge of the meanings of the prefixes are the key
here:
miria means ten thousands units
kilo means one thousand units ( 1 KILOmeter is equal to 1,000 meters )
hecta means one hundred units
deca means ten units
deci means a tenth part of the unit ( 1DECImeter is equal to 0.1
meters )
centi means a hundredth part of the unit
milli means the thousandth part of the unit
micro means the millionth part of the unit.

You can find a good tool to do conversions and to find conversion
factors at:
"ConvertIt.com Measurement Converter"
http://www.convertit.com/Go/ConvertIt/Measurement/Converter.ASP


More help to you will be found at:
"Washington State Department of Transportation
 - Metric Conversion Factors"
http://www.wsdot.wa.gov/Metrics/factors.htm#force


I did the exersice based in my own knowledge and find the related web
pages searching in Google with the search term "measurement
conversion" and the results page (you will find this very useful) is:
://www.google.com/search?hl=es&ie=ISO-8859-1&q=measurement+conversion&btnG=B%FAsqueda+en+Google&lr=


I hope this fill your needs in this question, but if you need a
clarification please post a request for it.

Best Regards
livioflores-ga

Thank you so much for the explanations and the web sites for the future.  Great job!