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I understood 1/6*1/6*5/6, and thus came up with 5/216. Of coz, then 1- 5/216.

But why multiply it into 3C1?!!!!! Pls explain. Thanks

You correctly calculated the number of ways that a 5 could come up in the first and second slots, or 5 5 X. However, there are two other ways you can get 2 fives. X 5 5 and 5 X 5. Hence, you need to multiple the number of ways to get a SPECIFIC pairing of 2 5's (which you did), by all the different ways to get 2 5's from 3 dice, which is 3C2 = 3.
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AkamaiBrah Former Senior Instructor, Manhattan GMAT and VeritasPrep Vice President, Midtown NYC Investment Bank, Structured Finance IT MFE, Haas School of Business, UC Berkeley, Class of 2005 MBA, Anderson School of Management, UCLA, Class of 1993

I understood 1/6*1/6*5/6, and thus came up with 5/216. Of coz, then 1- 5/216.

But why multiply it into 3C1?!!!!! Pls explain. Thanks

You correctly calculated the number of ways that a 5 could come up in the first and second slots, or 5 5 X. However, there are two other ways you can get 2 fives. X 5 5 and 5 X 5. Hence, you need to multiple the number of ways to get a SPECIFIC pairing of 2 5's (which you did), by all the different ways to get 2 5's from 3 dice, which is 3C2 = 3.

so it should be multiplied with 3c2, not 3c1 as stolyar says.

I understood 1/6*1/6*5/6, and thus came up with 5/216. Of coz, then 1- 5/216.

But why multiply it into 3C1?!!!!! Pls explain. Thanks

You correctly calculated the number of ways that a 5 could come up in the first and second slots, or 5 5 X. However, there are two other ways you can get 2 fives. X 5 5 and 5 X 5. Hence, you need to multiple the number of ways to get a SPECIFIC pairing of 2 5's (which you did), by all the different ways to get 2 5's from 3 dice, which is 3C2 = 3.

so it should be multiplied with 3c2, not 3c1 as stolyar says.