How many kgs of ruby's are there in 4 grams? Just divide by 1000 to get the answer of 0.004 kgs.
In 8.3 grams of diamonds there are 0.0083 kgs, to achieve this take 8.3 and just divide by 1000.
To convert 4.5 grams into kgs divide by 1000 giving 0.0045 kgs.
You've got 6.5 grams of platinum but someone only wants to buy that in units of kgs, to change the units divide by 1000. Now you have 0.0065 kgs.