The operation to perform:
 2,702 + 10,087
Rewrite the initial operation:
 2,702 + 10,087 =
10,087  2,702
Subtract the numbers:
10,087  2,702 = ?
Method used below: column subtracting, from right to left (traditional)
Stack the numbers on top of each other.
The ones digits line up in the first column from the right.
The tens digits line up in the next column to the left.
And so on...
Subtract column by column; start from the column on the right
Subtract the digits in the ones column:
7  2 = 5.
5 is the ones digit.
Write it down at the base of the ones column.
Subtract the digits in the tens column:
8  0 = 8.
8 is the tens digit.
Write it down at the base of the tens column.
Subtract the digits in the hundreds column:
0  7 = ?
The second digit is larger than the first.
Borrow from the next column to the left.
In this case we borrow across zero(s), a multiple step process:
Subtract 1 from the top digit in the column directly to the left.
But that digit is zero. You need to go further left.
Subtract 1 from the nearest column to the left with a nonzero top digit.
That is the column of the ten thousands: 1  1 = 0.
Cross out the top digit you've borrowed 1 from: 1.
Write the answer above that digit: 0.
Cross out the zero(s) that you were going across to the left: 0.
Write a 9 above each one of the crossed out zeros.
Why? In fact we have subtracted 1 from 10: 10  1 = 09.
When borrowing, 1 thousand = 10 hundreds.
Add 10 to the top digit in the column of the hundreds: 10 + 0 = ^{1}0.
After borrowing, the subtraction has become:
10  7 = 3.
3 is the hundreds digit.
Write it down at the base of the hundreds column.
Subtract the digits in the thousands column:
0 9  2 = 7.
7 is the thousands digit.
Write it down at the base of the thousands column.
Subtract the digits in the ten thousands column:
There is a single digit in this column: 1 0.
0 is the ten thousands digit.
Write it down at the base of the ten thousands column.
 0  9  ^{1}0   
 1  0  0  8  7 
   2  7  0  2 

 0  7  3  8  5 
Leading zeros
When leading zeros occupy the most significant digits of a natural number, they could be left blank and the numeric value stays the same:
07385 = 7385
Final answer:
 2,702 + 10,087 = 7,385