Given: 2605/91
First, find a multiple of 91 nearer to 2605: 90x30=2700,
so we can calculate 91x30 in our head: 91x30=(90+1)x30=2700+30=2730
Then, find the difference between 2730 and 2605. We will get: 125 (=2730-2605)
The difference,125, is greater than 91 (=91x1), and less than 182 (=91x2).
So, 2605 is less than 91x(30-1), and greater than 91x(30-2).
Therefore, the quotient is 28.
The remainder is calculated in our head as well.
91x28 is less than 2730 (=91x30) by 182 (=91x2),
but the difference between 2730 and 2605 is 125. So, the remainder is 57 (=182-125)
CK: 91x28+57=2548+57=2605 CKD.
Therefore,the answer is: the quotient is 28, and the remainder is 57.