For percentage related questions, assuming total share as 100 units works best for me
In this question we can assume that total share of votes is 100 votes
Assume, Y received "y" votes. Now it is given that X has received "20+y" votes
Since we have assumed that total votes are 100
(y+20)+(y) = 100
therefore, y = 40 and x=60
now comes the fun part!!
Since it is already given that "If Proposal Y received 70 votes". This means - the figure of 40 in our case represents an actual number of 70. (this could be a little hard to imagine at first but bear with me!)
Let's say 1 unit represents 100. With this analogy, how much does 30 units represents ?
we can simply use the unitary method to calculate 30 * 1 unit = 30 * 100
Similarly, here we can say "y=40" means 40 units which represents 70 actual votes
Therefore, x which was 60 units represents = (70/40)*60 actual votes = 105 votes
Hence, the difference between X and Y is 105-70 = 35 actual votes!