Robert drove to the beach on Friday, and back home on Sunday. What was his average speed on Sunday?

Question

His average speed on Sunday was 10 mph slower than his average speed on Friday. It took him 3 hours to make the trip Friday, and 4 hours for the return trip Sunday. What was his average speed on Sunday? 

Answer 1

the distance for each trip is the same and the distance is speed times time. So set up the equation where S is Sunday speed and F is Friday speed

 

4S=3F

and also we are told that 

S=F-10

 

now sibstitude the 2nd equation into the 1st. 

 

4(F-10)=3F

4F-40=3F

solving we have F=40mph

therefore S=40-10=30mph.

As a check be sure he travels the same distance each day (40)(3)=120=(30)(4)

 

it checks.  

Answer 2

Hello,

 

Excellent answer is already given by Karin S. I am adding just a shortcut solution. The given math problem translates into the system of 2 linear equations:

 

4S-3F=0

S-F=-10

 

It could be solved manually using either substitution or elimination methods.

 

Best regards,