We are given the following equation, and asked for which value we should complete it with so if becomes a TRUE equation (equality):
34 + 83 = 27 + ?
then, we combine the two numbers of the left of the equal sign:
(34 + 83 = 117)
117 = 27 + ?
now we subtract 27 from both sides to isolete the unknown number "?"
117 - 27 = ?
then the unknown number should be: 117 - 27 = 90
We can always check at the end that we answer is correct, by replacing the value we found in the original equation:
34 + 83 = 27 + 90
117 = 27 + 90
117 = 117
and we checked that both sides of the equal sign give the very SAME number.