Answer
$28+39=67$
Work Step by Step
Working left to right, add the corresponding digits in each place value. If the result is greater than 10, the digit in the tens position is carried over to the next higher place value position.
$28$
$\underline{39}\rightarrow$ $8+9=17$ Write 7 in the ones position. The 1 will carry.
$\underline{ }7\rightarrow$ $2+3+1=6$ Write 6 in the tens position
$67$