Statistics 1

The question is literally Q1 and I don't know how I have got this wrong:
http://www.mei.org.uk/files/papers/June13/S1_2013_June.pdf
Here's what I did:
n= 100
mean = 249
sum of x squared = 6240780
sum of x = 24940

so 6240780-(100*(249^2))
=40680
40680/99 = 410.9
root 410.9 = 20.27
They have done 24940 squared instead of n * Xbar squared
Any help on this as I have no idea why this is wrong at all!

Note that the calculation you've done and the one they've done is the same (the whole xbar squared or sigma x^2 ... bit) are equivalent.

However, you've used 249 instead of 249.4 and it's made a massive difference because you've squared it and multiplied by 100. Always use the ans button on your calculator.

Fair enough, but now i'm a tad confused, it says in the mark scheme you would get marked down for writing 249.4 as the mean as it is deemed as "over-specifying", so if they want 249 as the answer, why then use 249.4 in their next calculation?

Fair enough, but now i'm a tad confused, it says in the mark scheme you would get marked down for writing 249.4 as the mean as it is deemed as "over-specifying", so if they want 249 as the answer, why then use 249.4 in their next calculation?

Frequently they will ask for an answer to be rounded when asking for a specific value (eg give the mean) but for you to keep the "working value" for future parts of the question.

Common rule: don't round to the end and use as many digits as possible ti ensure accuracy. Only round when giving final answers!

That's kind of stupid, but okay!