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DRDO CSE 2022 Paper 1 | Question: 19

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To find the number of seven-digit numbers that are possible with exactly four 4s, you can use the stars and bars formula. This formula states that the number of ways to distribute n objects into k categories is:

(n+k-1) choose (k-1)

In this case, the number of objects is 7 (the number of digits in the number) and the number of categories is 2 (the 4s and the other digits). Therefore, the number of seven-digit numbers that are possible with exactly four 4s is:

(7+2-1) choose (2-1) = (8 choose 1) = 8

This means that there are 8 different seven-digit numbers that are possible with exactly four 4s.
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