Percentage question on voting In an election, 2 candidates participated.10% did not vote. 300 votes
  were declared invalid and the winner gets 60% of voting list and wins
  by 900 votes. Find no of valid votes.


Ans. 1500

What I tried:

Winner=60%;Not voted=10%;=>Loser=30%

ATQ:

(60-30)%=30%=900

=> Valid votes=90%-300$=\frac{90}{30}*900-300=2700-300=2400$

Where am I wrong?
Winner get 60% of the voting list and not total.

So it total votes are T, valid votes are T-300, and winner got 60% of that ie 0.6*(T-300)

Hope it clears your doubt.
Since the question goes discussing 2 candidates who participated in an election. 
In order to make it simple, we keep the total equal to 100. If 10% didn't vote that means 90% of people voted. The winner gets 60% of voting list so the looser must have got 40% of votes. 
100%-60%= 40%
The difference of percentages of the winner and the looser = 20%
Now, divide the number of votes got by the winner by the difference of their percentages to get total votes.
900/20*100= 4500 which is equal to total votes cast in the election. 
Once you've found the total votes look at the question again. It says that 10% did not vote, so find 90% of 4500 which  is 4050-300 because 300 votes were invalid and you'll get the answer= 3750
Answer=3750