18. There are four boxes of apples. By weighing any three of them, four different weights can be obtained: $70 \mathrm{~kg}$, $80 \mathrm{~kg}$, $73 \mathrm{~kg}$, $77 \mathrm{~kg}$. The weights of these four boxes of apples are $\qquad$ kg, $\qquad$ kg, $\qquad$ kg, $\qquad$ $\mathrm{kg}$.
Reference answer: $30,20,27,23$
Key points: Solving equations, equivalent substitution