$$1$$ similar skirt and $$3$$ similar blouses cost $$$75$$. If each skirt costs twice as much as each blouse, how much will each skirt cost? 
Overseas Competition->Knowledge Point->Word Problem Modules->Equation Word Problems->Equivalent Substitution in Equation Word Problems
1S + 3B = 75;  1S = 2B;  2B + 3B = 75;  5B = 75;  B = 75~$\div$~5 = 15  1S = 2~$\times$~15 = 30 