It took Brian 6 hours to drive to the sales conference. He took the same route on the way back home, but
thus time the trip took 7 hours. If Brian drove 9 mph faster on his way to the conference, how fast did he
drive on the way back home?

 54 mph

Step-by-step explanation:

d-the distance

s+9  -  the speed to the conference

d=6*(s+9)

s- is the speed on the way back home

d=7*s

so    6*(s+9)=7*s

       6s+54=7s

     -6s          -6s

       54= 7s-6s

       s=54  mph

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55

for this case we must apply distributive property to the following expression:

the distributive property states that:

then, when applying the property we have to multiply the -2 for each term within the parenthesis: