解：??$(1)1$??元紙幣的張數(shù)??$+5$??元紙幣的張數(shù)??$+10$元紙幣的張數(shù)??$=12$??張
??$ 1$??元紙幣的總面額??$+5$??元紙幣的總面額??$+10$元紙幣的總面額??$=79$??元
??$ 1$??元紙幣的張數(shù)??$=5$元紙幣的張數(shù)??$×4$??
??$ (2){{\begin{cases} { {x+y+z=12}} \\{x+5y+10z=79} \\{x=4y} \end{cases}}}$??
??$ (3){{\begin{cases} { {x=4}} \\{y=1} \\{z=7} \end{cases}}}$??
