How do you identify the information that is given to you that you dont need for solving a problem?

When reading a question, you need to be aware of the numbers presented and the facts. You need to pay attention to the question - what, when, where, how, why or how much. What is being asked in the question is always your clue. Unnecessary information will be eliminated once you understand the question.

Simple example from a Business book:

Jessica Fernandez, manager of Subway, had a bank balance of $5382.12 on March 1. During March, she deposited $60,375.82 received from sales, $3280.18 received as credits from suppliers, and $75.53 as a county tax refund. She paid out $27,282.75 to suppliers, $4280.83 for rent and utilities, and $12,252.23 for salaries and miscellaneous. How much did Fernandez deposit in March?

$60,375.82 (sales) + $3280.18 (credits) + $75.53 (refund) = $63,731.53
Fernandez deposit $63,731.53 in March.

The question immediately lead you to the sentence that is the key to the answer.
It is pretty obvious but try to recognize the important facts from the rest.
The question did not ask about Jessica's balance on March 1 or did not ask about how much she pay out. You can immediately eliminate that those information are not necessary to the problem being asked. The question is how much did she deposit in March. You need to find the sentence that points out the answer - During March, she deposited $60,375.82 received from sales, $3280.18 received as credits from suppliers, and $75.53 as a county tax refund. List all the numbers and add them together and you will come up with the figures she deposited. It is just a simple question but it presented the technique for the elimination process of unnecessary facts. It is not "we cannot see the answer" but the truth is we cannot see and understand the question.