Maths Olympiad 《Solve by Comparison and Replacement》 Pr$$3$$\&$$4$$ Question \#$$8$$  $$3$$ similar boxes of apples and $$5$$ similar boxes of oranges weigh $$\text{270kg}$$.  $$3$$ such boxes of apples and $$7$$ such boxes of oranges weigh $$\text{342kg}$$.  How heavy are a box of apples and a box of oranges? 
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