Find the smallest value of the sum of m and n so that the number 9m6n5 is divisible by 9                                                                                                                                            

Here, Sum of the known digits of 9m6n5 = 9 + m+ 6 + n +5 = 20+m+n

Now, the number before to 20, which is divisible by 9 is 18.

Thus, if we replace the m+n by - 2, the sum of digits will become 18 and the number becomes divisible by 9.

Hence, m+n is replaced by -2