Find a way to go home

Bob want to reach his house. He stands on a corner and his house is on another corner. To reach his house he has to pass many colorful block. How many way can he reach his house following 2 condition?

  • He can't go to left block.

  • He also can't go to down block.


The answer is 210.

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1 solution

Md Mehedi Hasan
Nov 26, 2017

Two possible way can be just like these picture:

One possible way is ( ) (\rightarrow \rightarrow \rightarrow \rightarrow \rightarrow \rightarrow \uparrow \uparrow \uparrow \uparrow )

Another possible way is ( ) (\rightarrow \rightarrow \uparrow \uparrow \rightarrow \rightarrow \uparrow \uparrow \rightarrow \rightarrow )

In every way we get 10 10 arrow where 6 6 are right arrow ( ) (\rightarrow) and 4 4 are up arrow ( ) (\uparrow) .

Finally we can arrange 10 10 arrow where 6 6 is same ( ) (\rightarrow) another 4 4 is same ( ) (\uparrow) .

So total way is = 10 ! 6 ! 4 ! = 210 =\frac{10!}{6!\cdot 4!}=\boxed{210}


Another solution is:

Total way = 10 C 6 = 10 C 4 = 210 =^{10}C_{6}=^{10}C_{4}=\boxed{210}

oh that's nice, I would never have thought of this

Vin Benzin - 3 years, 6 months ago

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