What you suggest would be finding, in effect, the value representing 70% of 28. (even though you converted the wrong number to a decimal) Not the same question at all. Besides, the result of multiplying 28 by .7 is 19.6, and where would one find .6 of a student, some students' poor study habits notwithstanding? Still another sanity check is that if 28 is 70% of something, then that something has to be larger than 28.
The question says: 28 is 70% of what? That means that you are looking for a value that when multiplied by 70% or 0.70 results in 28. A good placeholder for an "I don't know what" is x. So we need to translate the english sentence "70% of a value I don't know is 28" into algebraic symbols.
'70%' means 0.70
'of' means multiply
'a value I don't know' means x
'is' means =
and 28 means 28
Now, in order to get x by itself on the left so that we know its value, we need to divide by 0.70