How many ways are there to make $$$80$$ using some combination of $$$5$$, $$$10$$ and $$$20$$ notes? 
Overseas Competition->Knowledge Point->Word Problem Modules->Unitary Method Problems->Finding the Total Value using Unitary Method
Condition on the number of $$$20$$ note you use.  $$4$$ gives $$1$$ possibility,  $$3$$ gives $$3$$ possibilities,  $$2$$ gives $$5$$ possibilities,  $$1$$ gives $$7$$ possibilities,  $$0$$ gives $$9$$ possibilities. 