Question 971920
With replacement, it is (5/12)(4/11)(3/10)=(60/1320) for white; for black,it is (7/12)*(6/11)*(5/10)=(210/1320).  This product is [60*210]/[1320*1320]=(1/22)(21/132)=(1/22)(7/44)=7/968.  Note:  this is misleading, for with replacement means only replacing after the three white balls are drawn, which is not how it may be interpreted.

Without replacement, it is (5/12)(4/11)(3/10)(7/9)(6/8)(5/7)=(60/1320) (210/504)=(1/22)(210/504)
=(1/11)(105/504)=(1/11)(35/168)=35/1848

2.  60 and 65 = 0.7*0.4*0.8=0.224; 60 and 70 0.7*0.6*0.2=0.084; 65 and 70= 0.3*0.4*0.2=0.024; all 3 0.7*0.4*0.2=0.056.  The sum is 0.338.  The important thing to do here is to make sure you multiply by all three probabilities.  The probability of two is straightforward, but if it isn't all of them, then you have to multiply by the probability the third person WON'T be part of it.




There are 8 people.  
If x is in place 1 y has 1 places to be out of 7
x is in place 2, y has 1 places
etc. In every place x may stand, y can stand somewhere.  The key is that it is THREE people, not at least 3 people.
This is 8/56=1/7