In how many different ways can you can write the word "Izumi" if you start with the red central letter 'I'?
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Ah, I initially thought it was 4 × 3 × × 3 × 3 , because each time you move right there are 3 options to take. That might be a common mistake that others make.
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To tell you the truth, I did the same mistake when I was counting it for the first time :)
We can use the classical "walking on a rectangular grid" problem to solve:
This is a quarter of the image. 'Cause the image is symmetric, we can calculate in a quarter and then multiply it by 4 .
Starting from letter 'I', we need 4 steps to reach the last 'I'. In each step, there are 2 ways: up or right.
So the number of 'IZUMI's are 2 4 = 1 6 . Multiply by 4 gives us 6 4 .
But, 4 words 'IZUMI's are duplicate, which are, the vertical and horizontal ones. We need to subtract by 4 .
The answer is 6 0 .
vertical = v
horizontal = h
v or h : 1
vh : 3
hv : 3
vhv : 3
hvh : 3
hvhv : 1
vhvh : 1
sum = 15
total = 15*4 = 60
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Solution:
Go on letter 'Z' to the left. From there, you can go either left, down or up, on a letter 'U'.
If left 'U' is chosen, there are 3 different ways to finish word if 'M' to the left is chosen, if down 'M' is chosen there are 2 ways, that is the same with upper 'M'. Meaning, 7 ways for that left 'U'.
Now, if upper 'U' is chosen there are 4 ways to finish it. It is completely similar for down 'U'. Both of them united, 8 ways.
Since all previously mentioned ways started with left 'Z' (15 ways), solution is obtained when multiplied with 4 (since it is the same if down, up or right 'Z' is chosen). The answer is 6 0