Mind Boggling BALLS....

There are unlimited number of identical balls of four different colours. How many arrangements of at most 8 balls in a row can be made by using them?

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The answer is 87380.

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3 solutions

4+4^2+4^3+4^4+4^5+4^6+4^7+4^8=87380.

Jeffrey C
Jun 18, 2014

Split the problem into cases. The cases would be 1 ball in a row, 2 balls in a row, 3 balls in a row, and so on. There are 4 possible colors for each ball in the row. Therefore, there are 4 ways for the 1 ball in a row, 4x4 or 16 ways for the 2 balls in a row, 4x4x4 ways or 64 ways for the 3 balls in a row, and so on. Therefore, there are a total of 4+16+64+256+1024+4096+16384+65536=87380 ways.

Krit Phuengphan
Jun 4, 2014

I got the final result as following, the answer is a = 1 8 ( b = 1 4 ( 4 b ) ( a ! a 1 ! a 2 ! a 3 ! a 4 ! ) ) , \sum_{a=1}^{8}\begin{pmatrix}\sum_{b=1}^{4}\begin{pmatrix} 4 \\ b \end{pmatrix}\begin{pmatrix}\frac{a!}{a_{1}!a_{2}!a_{3}!a_{4}!}\end{pmatrix}\end{pmatrix}, where b a b\leqslant a ,

a 1 + a 2 + a 3 + a 4 = a a_{1}+a_{2}+a_{3}+a_{4}=a ,

and i b i\leqslant b , for a i a_{i} .

Unfortunately, I cannot express it to be a more explicit form. I, however, obtained the answer as 87380 \boxed{87380} which equal 4 ( 1 4 8 ) 1 4 \frac{4(1-4^8)}{1-4} .

PS. I'm not sure whether the answer should be 87381 since we are considering arrangements of at most 8 balls in a row, is zero-ball arrangement in?

4+4^2+4^3+4^4+4^5+4^6+4^7+4^8=87380.

Chandrachur Banerjee - 6 years, 12 months ago

87381 was one of the numbers I tried too. As a matter of fact, the accepted answer is probably wrong because there is no such thing as one ball in a row, is there? In other words, the minimum number of balls should be two. But of course I'm splitting hairs.

Bill Bell - 6 years, 9 months ago

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