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There are 20 intermediate stops on a route of a transport corporation bus. The number of ways in which the bus can stop at 6 of these intermediate stops such that no 2 stops are consecutive is ?
ago in Combinatory by (9 points) | 27 views

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Number of stopping stations = 6

Number of non-stopping stations = 14

If we lay down the 14 non-stopping stations then we will have 15 choices where the stopping stations can be and none of them would be consecutive.

* 1 * 2 * 3 * 4 * 5 * 6 * 7 * 8
* 9 * 10 * 11 * 12 * 13 * 14 *      

Here * are free positions and numbers are non-stopping stations. 

So we have to select 6 places from 15,

i.e., $${15\choose6} = \frac{15\times14\times13\times12\times11\times10}{6\times5\times4\times3\times2\times1} = 5005$$

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