remember that the second time you take a ball out there are less balls in the box i.e the first time you take a ball out the chance of it being black is 5/8 but now there are 4 black balls left and 7 balls left in total.
sorry, cubes
hai every body... i still stuck until now for the following problem. hope somebody can guide me.
Question
A box contains 5 black cubes and 3 red cubes. Two cube are drawn at random from the box. find the probability that :
(a) both cubes are black
(b) both cubes are the same colour
(c) both cubes are different colours
ans given:
5/14, 13/18 ( i also confuse this ans for which question (a)/ (b) or (c)
My solution:
(a) P( both cubes are black ) = P (Black,Black)
= 5/8 * 5/8
= 25/64
(b) P (both cubes are the same colour) = P (Black,Black) + P (Red,Red)
= 25/64 + 3/8*3/8
= 25/64 + 9/64
= 34/64
= 17/32
Remarks: base on my calculation it look like it wrong when i compare it with the given ans..
base on your info this is my ans:
(a) P( both cubes are black ) = P (Black,Black)
= 5/8 * 4/7
= 5/14 (same as per ans)
(b) P (both cubes are the same colour) = P (Black,Black) + P (Red,Red)
= 5/8 * 4/7 + 3/8*2/7
= 13/28 ( not same with ans given)
(c) both cubes are different colours
= P ( Black,Red) + P( Red,Black)
= 5/8 * 3/7 + 3/8*5/7
= 15/28 ( not same with ans given)
any one can confirm my ans