Answer
$428+21=449$
Work Step by Step
Working left to right, add the corresponding digits in each place value.
$\ \ \ 428$
$\ \ \underline{+21}$
Add the ones position $8+1=9$
Add the tens position $2+2=4$
Add the hundreds position $4+0=4$
Write the result of each addition step in its proper position for its place value.