Answer
$819+21=840$
Work Step by Step
Working left to right, add the corresponding digits in each place value.
$\ \ \ 819$
$\ \ \underline{+21}$
Add the ones position $8+1=10$
Carry over the digit in the tens place.
Add the tens position $1+2+1=4$
Add the hundreds position $8+0=8$
Write the result of each addition step in its proper position for its place value.