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Final Review Q16 (rounding decimals)

Geoffo66

Topic Author

Geoffo66 created the topic: Final Review Q16 (rounding decimals)

Hi All,

I am working through all the exercises and review questions and find that rounding up or down becomes critical...in the question listed above for example.

My final calculation for the ZFW was 2628kg and 706.63 index units. When you put this into the flow chart the shift is 8 and the final ballast fuel weight works out to just over 23kg...answer a. was 25kg so close enough I pick that one.

After checking the answer it is d. 38kg...how could I be that far out? The only difference in my calcs was that the ZFW index units in the answer is 706.64, so using this in the flow chart the shift is now 13 and the final ballast weight is 38kg.

Trying to find where this 0.01 crpet in was in the index unit calc for row 1 (154 x 2290 / 10000 = 35.266) I used 35.26 where the answer must have used 35.27.

From this example the rounding becomes quite critical and could be the difference between a pass or fail as they are not checking your working just the final answer. Does anyone have any tips or the best method to adpot??

bobtait replied the topic: Re: Final Review Q16 (rounding decimals)

Geoff

The workings in the book are based on taking the index units to the nearest first decimal place. So 154 in row 1 would be 35.266 which would be rounded to 35.3. The examiner has said some time ago that he would be happy with that. People who have used that technique have been able to get good pass marks in the exam.

In fact, when you think of it, the whole idea of using a moment index is to reduce the number of digits that have to be managed. You could even argue that converting a moment to a moment index and by dividing by 10000 and then introducing one decimal place is a bit of a contradiction. Never the less, if I were you, I'd stick to working to the nearest first decimal place of an index unit.

Geoffo66 replied the topic: Re: Final Review Q16 (rounding decimals)

Thanks Bob,

I aggree with your reasoning, but once you have found the total moment then multiply by 10,000 again to find the CoG position, I find that is where it can make some difference as it affects the shift amount. I did my exam yesterday and adopted your philosophy and got through with 83% so all good...a tough exam, there is no time for thinking about how to attack the questions you have to know your stuff! The books have been a great help!