Two plums and one cherry weigh $$80\text{g}$$. One cherry and one plum weigh $$50\text{g}$$. What is the weight of four plums? 
Overseas Competition->Knowledge Point->Word Problem Modules->Equation Word Problems->Equivalent Substitution in Equation Word Problems
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 $$p+c=50$$. We can deduce that $$p=80-50= 30$$, and so four plums weigh $$120 \text{g}$$.  Revise from ($$2017$$ Primary Mathematics Challenge-February, Question \#$$18$$) 