I've got to hand this in for tomorrow. No problems. But I can't do standard deviation!

It's these online worksheets, and the teacher expects them to be done. I have tried my hardest to work it out revision guides, etc. No help

There are four questions:

1) A teacher records results on the length of time taken to do a test:

24, 18, 21, 17, 25, 19, 23, 17, 25, 15

Find the mean: Find the S.D

Once they have completed the test, each pupil sits a mental arithmetic test, which takes 5 mins.

Write the New Mean Write the new S.D

2) 23 archers take part in a competition, they get a score out of 600. The sum of their scores is 11 362 the sum of the squares of the scores is 5645713

Find the mean: Find the S.D

One archer's score is incorrectly recorded as 341. He actually scored 346

Find the new mean Find the new S.D

3) Afolabi goes swimming twice a week. He records how many lengths he swims each time here are the results

Lengths Frequency

20 5

22 9

24 19

26 28

28 26

30 13

Find the mean length Find the S.D.

The next time he goes swimming he does 9 lengths. How does this affect the S.D? Increase. Decrease. Can't tell

4) A teacher asks her closs how long they spend doing their homework.

Time Freq

0<t<=10 1

10<t<=20 3

20<t<=30 6

30<t<=40 16

40<t<=50 2

50<t<=60 2

Find the mean. Find the S.D

A student was absent when the results were recorded. She spent from 30-40mins doing her homework

Find the new mean Find the new S.D

Please help :-( I'm finding this really hard!