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.
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Two possible way can be just like these picture:
One possible way is ( → → → → → → ↑ ↑ ↑ ↑ )
Another possible way is ( → → ↑ ↑ → → ↑ ↑ → → )
In every way we get 1 0 arrow where 6 are right arrow ( → ) and 4 are up arrow ( ↑ ) .
Finally we can arrange 1 0 arrow where 6 is same ( → ) another 4 is same ( ↑ ) .
So total way is = 6 ! ⋅ 4 ! 1 0 ! = 2 1 0
Another solution is:
Total way = 1 0 C 6 = 1 0 C 4 = 2 1 0