bhandariavi
In city A, the streets are aligned in a grid (see attachment), where the east-west roads are called 1st Rd, 2nd Rd, 3rd Rd, etc, increasing in number as one moves northward. The north-south roads are called 1st Ave, 2nd Ave, 3rd Ave, etc, increasing in number as one moves eastward. There is a park that runs from 5th Ave to 7th Ave and from 3rd Rd to 5th Rd, as pictured. If Bill needs to walk from the corner of 2nd Rd and 3rd Ave to the corner of 6th Rd and 8th Ave in the shortest possible time without walking through the park, how many different routes could he take?
1. 45
2. 54
3. 66
4. 98
5. 19
There must be some short cut ways to solve this problem with combination factorial formula.
Can anyone shed light on this?
You might have seen a simpler problem without the park. You need to take shortest possible time so you cannot retrace any path. You cannot go down or left. You need to go up and right only.
To go from bottom left corner to top right, you will have to go 4 paces Up and 5 paces Right.
You can do this in many ways e.g. UUUURRRRR, UURRRUURR etc
There will be total 9!/(5!*4!) = 126 ways
Attachment:
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From these we need to remove those ways which use the red paths because they pass from the park. Note that you need to reach one of the thick red paths first. Only then can you reach either one of the two thin red paths. So if we just remove the cases using the thick Red paths, we will be done.
In how many ways can you go from bottom left to top right using the thick Up path?
You will need to take 3 Rights and 1 Up (in any order), then the Up thick Red path, then either (Up thin Red path, 2 Rights and 1 Up) or (Right thin Red path, 2 Ups and 1 Right)
You can do this in \(\frac{4!}{(3!)} * 1* 2 * \frac{3!}{2!} = 24\) ways
In how many ways can you go from bottom left to top right using the thick Right path?
You will need to take 2 Rights and 2 Ups (in any order), then the Right thick Red path, then either (Up thin Red path, 2 Rights and 1 Up) or (Right thin Red path, 2 Ups and 1 Right)
You can do this in \(\frac{4!}{(2!)(2!)} * 1* 2 * \frac{3!}{2!} = 36\) ways
Now all we need to do is: 126 - 24 - 36 = 66 ways