Hi there. The interviewer will only hand you the data asked for. So it's crucial to amke sure you know exactly what you're doing before digging into the numbers.
When I do math question, a backward deduction has worked very well. I always follow below steps. Let's assume the interviewer asks you to calculate the profit the company will make next year.
Step 1: State your general logic - "In order to calculate the profit the client can make next year, we can use projected revenue deduct projected cost"
Step 2: Breakdown the big blocks - "So revenue can be calculate by mutiplying price and vol, while cost can be calculated through xxx plus xxx"
Step 3: Ask for data - "I'm wondering whether there is data for xxx, xxx, xxx available?"
Step 4: Do the math - work on the numbers given as fast and accurate as possible
Step 5: Sanity check
As you can see from above example, if you have your general logic sorted out before asking for any data, you can avoid 1). getting confused on which data to use if given a long list of numbers 2). missing any data
Hope this makes sense!