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}}}$?
