Sum means add
the number is x
13 divided by a number is 13/x
the number divided by the number is x/13
so

also can be simplified to

or

translated it is

where x is the number
X would equal 2.7 //////////////
D divide by 1000… you are shifting the decimal 3 places (there are 3 zeros in 1000) you are dividing because the number is getting smaller.