"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
