The answer is 7C4.
Searching for 60, we encounter 4 keys less than 60 (10,20,40,50) & 3 keys greater (70,80,90).
The four lesser keys must appear in ascending order while the three greater ones must appear in descending order otherwise some keys will be left out in the traversal.
Note that in the traversal, these lesser keys might not be continuous and can be separated by greater keys.
For eg: 10, 90, 20, 30, 80, 40, 70, 50
but the order of both groups of keys (lesser and greater) remains the same individually.
Now, out of total seven positions, the lesser keys acquire four positions which can be selected in 7C4 ways. Once we know the places of these four keys, the places of the three greater keys only have one permutation.
For eg: if we know that
10, _, 20, 30, _, 40, _, 50
Then there's only one permutation of places for 90, 80 & 70 which is place number 2, 5 & 7 respectively.
Therefore for each combination of lesser keys, there's a unique combination of greater keys.
So total combinations= 7C4*1 =35 ways
This ans was given on stack overflow, it crystal clear explanation.