Factors of many

How many factors does ( 120 6 ) \left( \begin{matrix} 120 \\ 6 \end{matrix} \right) have?

Notation : ( M N ) \binom MN denotes the binomial coefficient , ( M N ) = M ! N ! ( M N ) ! \binom MN = \frac{M!}{N!(M-N)!} .


The answer is 768.

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1 solution

Joel Yip
Apr 16, 2016

We can show that

( 120 6 ) = 120 6 ( 120 1 1 ) ( 120 2 1 ) ( 120 3 1 ) ( 120 4 1 ) ( 120 5 1 ) \left( \begin{matrix} 120 \\ 6 \end{matrix} \right) =\frac { 120 }{ 6 } \left( \frac { 120 }{ 1 } -1 \right) \left( \frac { 120 }{ 2 } -1 \right) \left( \frac { 120 }{ 3 } -1 \right) \left( \frac { 120 }{ 4 } -1 \right) \left( \frac { 120 }{ 5 } -1 \right)

120 6 ( 120 1 1 ) ( 120 2 1 ) ( 120 3 1 ) ( 120 4 1 ) ( 120 5 1 ) = 20 × 119 × 59 × 39 × 29 × 23 \frac { 120 }{ 6 } \left( \frac { 120 }{ 1 } -1 \right) \left( \frac { 120 }{ 2 } -1 \right) \left( \frac { 120 }{ 3 } -1 \right) \left( \frac { 120 }{ 4 } -1 \right) \left( \frac { 120 }{ 5 } -1 \right) =20\times 119\times 59\times 39\times 29\times 23

20 = 2 2 × 5 20={ 2 }^{ 2 }\times 5

119 = 7 × 17 119=7\times 17

39 = 3 × 13 39=3\times 13

the other are all prime numbers so,

2 2 × 3 × 5 × 7 × 13 × 17 × 23 × 29 × 59 { 2 }^{ 2 }\times 3\times 5\times 7\times 13\times 17\times 23\times 29\times 59

3 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 = 768 3\times 2\times 2\times 2\times 2\times 2\times 2\times 2\times 2=\boxed { 768 }

Moderator note:

Simple standard approach.

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Aditya Kumar - 5 years, 1 month ago

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