You want to do to one side the same thing you do to the other side.
In order to get the n by itself you divide the 25n by 25. But in order to not change the answer you need to do the same to the 70.
so n=
which can be reduced to 
so n=
In order to get the n by itself you divide the 25n by 25. But in order to not change the answer you need to do the same to the 70.
so n=
so n=