Soln is given by using nCr.. How to know that nCr will be used here..
And soln without using nCr?
For second method, we have 3 odd nos. out of which two are placed at ends therefore 3C2 and remaining 5 digits out of which 2 are to be used.
Hence numerator- 3C2*5C2 and denominator 7C4. First method is easier as it is simpler form of the second method.