the first one you got it correct. you've got the right equation, just takes a little working. you should be able to do it in your head in about a minute.

average is 9, subtract from each, you get 6,3,0,9, square each and add, you get 126, divide by n-1. but I do see one problem, the answer should be 9,42 not 9,31.5. to get 31.5, you divide by n, or, 4. to get the standard variance, you divide by n-1, which would be 3. 126/3 is 42...

as for the second problem, this is the difference between a permutation and a combination.

for a permutation, the order you choose your items doesn't matter. this is the case with the question you're asking. it does not matter how you select the two books, you just select them. the eqtn for this is n!/(n-r). it would be 6!/4! which is 6X5, or, 30.

a combination is a little more tricky and difficult to spot. this is when the order you choose your items in DOES matter. for example:

you have 3 red balloons, 4 yellow, and 5 blue, how many ways can you choose 3 balloons?

so the eqtn for a combination is what you have, n!/[(r!)(n-r)!]. that extra r! in the denominator makes all the difference. so the answer to this question would be 12!/[3!(9!)], or, 220... without that extra r!, you'd get 1320 which is the wrong answer.

GOOOOOD LUCK!!!

oh and if you can help me out, I'm looking for what the DAT is talking about when it asks for Descriptive and Experimental embryology. particularly implanting certain cells into a chicken or rat embryo and having the animal grow weird stuff... where do I learn this **** from?!

by the way, my exam is on the 30th... :-(