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Probability Level pending

How many numbers greater than 5000 5000 can be formed by using the digits 2 , 3 , 0 , 4 , 7 2,3,0,4,7 and 6 6 . Repetition of digits is not allowed.


The answer is 1320.

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1 solution

The desired numbers are of the following three mutually exclusive types.

(a) Numbers of six digits each.

(b) Numbers of five digits each.

(c) Numbers of four digits each.

For type (a) the left-most place can be filled in just 5 5 ways (with 3 , 6 , 7 , 4 3,6,7,4 or 2 2 ). Then the next place in 5 5 ways because 0 0 is now eligible; etc.; the number of these numbers is 5 × 5 × 4 × 3 × 2 × 1 5 \times 5 \times 4 \times 3 \times 2 \times 1 or 600 600 numbers.

Similarly, there are 600 600 numbers for type (b).

For type (c) the left-most place can be filled in 2 2 ways (with 6 6 or 7 7 ). Then the next place in 5 5 ways; etc.; the number of these numbers is 2 × 5 × 4 × 3 2 \times 5 \times 4 \times 3 or 120 120 numbers.

Finally, the number of numbers of all types is 600 + 600 + 120 = 600+600+120= 1320 \color{#D61F06}\boxed{1320}

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