"90 is 1/10 of what?"
"of what" could be translated to "a number" which could be translated to x, so:
let x = the number = of what
"1/10 of" tells us we need to multiply 1/10 by x
1/10x
"90 is" is the same as
90=
so put them together to get:
90=1/10x
multiply both sides by 10 to get
x=900