Four buildings - A , B , C and D are connected with different paths. You are currently standing at A .
How many ways are there for you to get to D and back to A ?
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F o r A − > B − > C − > D : t h e r e a r e 3 × 3 × 2 = 1 8 w a y s F o r A − > C − > D : t h e r e a r e 1 × 2 = 2 w a y s F o r A − > D : t h e r e a r e 1 w a y s = 1 w a y F o r t o t a l t h e r e a r e : 1 8 + 2 + 1 = 2 1 w a y s f r o m A − > D F o r b a c k D − > A t h e r e a r e t h e s a m e 2 1 w a y s S o f r o m A − > D a n d t h e n D − > A : t h e r e a r e 2 1 × 2 1 = ⌈ 4 4 1 ⌉ w a y s
That's Right !
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First of all we check out the number of ways from A to D .
From A to C via B there are : 3 × 3 = 9 ways
Hence, from A to C there are: 9 + 1 = 1 0 ways
Now, from A to D via C and B we have: 1 0 × 2 = 2 0 ways
Hence, from A to D there are: 2 0 + 1 = 2 1 ways
Now, Similarly we'll have 2 1 ways back to A .
Thus, there will be 2 1 × 2 1 = 4 4 1 ways from A to D and back to A .
ANSWER : 4 4 1