Finding the missing digit of a number.

Subtract 6 from 3Q784 = 3Q778    then Q must be 2

Proof 3642*9 + 6 = 32784

A number is divisible by 9 if the sum of its digits is divisible by 9.

Can you find a digit Q such that  3 + Q + 7 + 8 + 4  is a multiple of 9? 

Oh yes, the nine's remainder rule! So the answer is five right?

Because: $${\mathtt{3}}{\mathtt{\,\small\textbf+\,}}{\mathtt{7}}{\mathtt{\,\small\textbf+\,}}{\mathtt{8}}{\mathtt{\,\small\textbf+\,}}{\mathtt{4}} = {\mathtt{22}}$$ and 27 is the next number divisible by nine, so five is the difference.

Oops, that wasn't you question; sorry!

First, subtract 6 from 3Q784 and then sum those digits to get a multiple of 9.

Remainder of 6 then...24...right?