Answer:
Plan A lasts 0.5 minutes
Plan B lasts 1.5 hours
Step-by-step explanation:
Let Plan A last [tex]a[/tex] hours, and Plan B last [tex]b[/tex] hours.
We have
[tex]\left \{ {{3a+5b=9} \atop {12a+2b=9}} \right.[/tex]
We'll multiply the top equation by 4, then subtract the second equation from it to get
[tex]18b=27\\b=1.5[/tex]
We'll substitute this back into the original first equation [tex]3a+5b=9[/tex] to get
[tex]3a+7.5=9\\3a=1.5\\a=0.5[/tex]
[tex]\therefore \left \{ {{a=0.5} \atop {b=1.5}} \right.[/tex]