I tried to distribute the 4 biggest number ($38+20 + \_ = 32+23 + \_$ ) into the boxes first. Then I placed $8$ and $11$, respectively, into the empty boxes left. Finally, I was left with the number that doesn't fit: $18$.
Luckily this worked for me, but I don't know what would I do if the "unused" number were one of the biggest numbers.
What is the technique to tackle this problem considering this is a problem given to high school students?