Question 978018
Q3:  Two red marbles can occur on the first or second draw or first and third, 10 ways altogether 5C2
The first probability is (5/18) the second is (4/17).  The white marbles also have 10 ways they can be chosen (5C3=5C2)  (6/16)(5/15)(4/14)
The denominators are always 18*17*16*15*14.  The numerators are always 5*4*6*5*4.  There are 10 ways to place the red balls.  Once they are placed, the white balls have no choice.  So there is one 10 in the numerator, not 2.

10*2400/18*17*16*15*14=0.0233  A

Q2:Probability of at least 2 = 1- probability of 1-probability of 0.
Probability no defective is (16/20)(15/19)(14/18)(13/17)(12/16)=0.0188
Probability of 1 defective is (16/20)(15/19)(14/18)(13/17)(4/16) *5  The defective typewriter can be 1st-5th, but the denominators stay the same, There are 5 ways it can happen.
This probability is 0.4696

The total of the two is 0.4884
Subtract from 1, and the probability of at least 2 is 0.5116.