Permutation, Combinatorics Stuck here : there are 100 objects labeled 1, 2,...100. They are arranged in all possible ways. How many arrangements are there in which object 28 comes before object 29.

My approach : Consider object 28 &amp; object 29 , a single object. Now we have a total of 99 objects which can be per mutated in 99! ways . 
But the answer is 4950*98! .

What's wrong with my approach?
Your answer would be right if the question asked for number of arrangements where 28 comes exactly before 29. Here, 28 can come anywhere before 29. So obviously answer would be greater. For the exact answer, note that 28 and 29 are equally likely to come before the other one. Moreover, both cannot occur together (this is obvious here, but in general it is necessary to keep this in mind). So the correct answer = 100! / 2, which can also be written as 4950 * 98!
You can take object 28 and 29 as alike. So the permutation order i.e. 28 before 29 is not disturbed. This gives no. of arrangements as 100!/2! = 98!*99*100/2=98!*4950