Let's recap some helpful facts that will help us to convert fractions.
A thousandth is a tenth of a hundredth.
So 10 thousandths make 1 hundredth.
A hundredth is a tenth of a tenth.
So 10 hundredths make 1 tenth.
And 100 thousandths make 1 tenth too.
Using this knowledge, we can then say,
4/100 = 40/1000
3/10 = 300/1000
850/1000 = 85/100
To multiply by 10, we move the digits one column to the left.
To multiply by 100, we move the digits two columns to the left.
To multiply by 1000, we move the digits three columns to the left.
To divide by 10, 100 and 1000, we do the same steps as above, except the digits move to the right instead of left because the number gets smaller when we divide.