I'm new here and pretty old so I may be wrong, but I do believe it has to do with decimals. Research it a little.

640/1600

A trick that I have found very useful in division problems that have a smaller number being divided by a bigger number (or where you know there will be a decimal in the answer), is to divide by adding as many zeros as necessary to the end of the smaller number so that it is of more value than the bigger number.
For instance, in this case, you would add one zero to the end of the 640 (smaller number), resulting in 6400. From there, you do 6400 divided by 1600.
But you must take into consideration that every time you ADD a ZERO, you must add a zero for the decimal place in the answer. For example, 6400/1600 is equal to 4, but because you added one zero, the answer would be 0.4
Another example would be if you did 64/1600. Because you add two zeros to the end of the 64, your answer would be 0.04
I apologize if this is a tad bit confusing. Hopefully it helps.