The papers of 4 student can be checked by any one of the 7 teachers. If the probability that all the 4 papers are checked by exactly 2 teachers is A, then the value of 49 A must be

Total number of ways in which papers 0f 4 students,
can be checked by seven teachers = 7⁴
Now, choosing two teachers out of 7 is 7C₂ = 21
The number of ways =  7C₂ (2⁴-2)=(21) (14)
Required probability $=\frac{\left(21\right)\left(14\right)}{7^4}=\frac{6}{49}=\mathrm{A}$