Chapter 4 - Section 4.4 - Systems of Linear Equations and Problem Solving - Exercise Set - Page 320: 27

plane speed: 455 mph, wind speed: 65 mph

into wind: $(x-y)*2=780$ with wind: $(x+y)*1.5=780$ $(x-y)*2=780$ $(x+y)*1.5=780$ $(x-y)*2=780$ $(x-y)*2/2=780/2$ $x-y=390$ $(x+y)*1.5=780$ $(x+y)*1.5/1.5=780/1.5$ $x+y=520$ $x-y=390$ $x+y=520$ $x+y+(x-y)=520+390$ $2x=910$ $2x/2=910/2$ $x=455$ $(x-y)*2=780$ $(455-y)*2=780$ $(455-y)*2/2=780/2$ $455-y=390$ $y=65$