In Desperate Need Of Help! Midterm Due Tomorrow

I am a first time user so I have not had time to browse other threads or anything. Anyways, I am in dire need of help, as I have a midterm due tomorrow by 9:00 am. Our teacher sprung it on us unexpectantly today in class, giving us a ten page midterm to do over night. i've been working on it since about 5 oclock and am finding i have a lot of difficulty.

Here is what I am having trouble with:

1) The difference between standard deviation and variance. If asked to find the standard deviation of something, what formula do you use compared to the one used if asked to find the variance? (My book is very unclear as to which formula belongs to which, so Im unsure which oneto use for questions.)

the frequency distribution of grades on a test for a class of 30 students is given below:
GRADES frequency
51-60 4
61-70 6
71-80 8
81-90 10
91-100 2

Find the variance of grades.
Find the standard deviation of the grades.

2)Frequency Distribution: there are two questions relating to frequency distribution and ask me to find D7 and P30. while I know they have to do with dividing the answer up into different section i am very confused. The chart i am given is:
weight lbs frequency
100-120 3
121-140 8
141-160 10
161-180 9
181-200 10

then it asks to find D7 and P30.

3) We are given these numbers:
115, 120, 136, 140, 144, 155, 160, 182, 190, 200.

Find the hinges of this data.
Constuct a boxplot.
Use the hinges as quartiles and find the interquartile range.

Any help on these problem would be greatly appreciated, as there are approximately 9 more just like them and i am in the dark of how to do them.I've read the book, checked my notes, as well as look at other websites and am still very confused. PLEASE HELP. even if you are not willing to give me the answer if your conscience finds it "cheating" anything, and i mean anything, will help me.

Please email me or reply to this thread if you wish to help.