Two plums and one cherry weigh $$80\text{g}$$. Two cherries and one plum weigh $$70\text{g}$$. What is the weight of four plums? 
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Let the mass of one plum and the mass of one cherry be $$p \text{g}$$ and $$c\text{g}$$ respectively.  Then $$2p +c=80$$ and $$2c+p=70$$. Hence $$3p +3c=150$$, and $$p+c=50$$. But since $$2p+c=80$$, we can deduce that $$p=80-50= 30$$, and so four plums weigh $$120 \text{g}$$. 