What I tried:
1st phase of voting:
Valid votes=900;invalid votes=100
Let no. of people who voted for(supporters)=x
Let no. of people who voted against(opponents) =y
2nd phase of voting:
Valid votes=800;invalid votes=200
The new number of opponents=$\frac{3}{2}y$, which is now a majority.
Which is equal to$=x(1+\frac{300}{100})=4x$ $$ 4x=\frac{3}{2}y$$ $$\frac{x}{y}=\frac{3}{8}$$
Which seems to contradict the statements.
I took the statement to mean that the margin for rejection in the second vote was $300\%$ more than the margin for acceptance in the first vote.
Thus if $y$ initially were against, then $900-y$ were initially for. The motion passed by a margin of $(900-y)-y=900-2y$.
In the second vote, $1.5y$ voted against, and $800-1.5y$ voted for. The motion failed by a margin of $1.5y-(800-1.5y)=3y-800$.
So $3y-800=4(900-2y)$.
This gives $y= 400$.
Initial vote: Against $400$; For $500$.
Second vote: Against $600$; For $200$.