Hi Abeesh, 

Even after narrowing down the options to just 11 and 20, you would still have to follow the procedure in order to reach the final answer. 
Yes guessing can narrow down the options you need to check, and this can be useful in questions where you can put in the values and check. But this question was fairly simply and had negative marking for a wrong answer, giving some time to the question and reading it carefully would have given you the correct answer. 

Please feel free to ask any more question you might have. 
Regards

Hi, from the question, we can clearly infer that one of the people in the group weighs 53kgs and one of the person weighs 57 kgs.

12 should be the maximum possible number of people who can board the lift. Ex: 4*53+7*54+57=647

Sorry, my bad..the limit is 630.

Hi,

It is not stated that only one person is the lightest among the group. Multiple people can also weigh least and can be termed as lightest.

Hi Vishisht Kurmi,
When you are considering N/2 {53+57} = 630
You are saying that the terms in the series would sum to 110(53+57).
In few cases this may go wrong. So, it is better to consider the minimum possible value for all the remaining people. That will give you the correct answer.
I hope this helps!!