SOLUTION: Super confused, need help by today- Lisa travels at 30km/h on the way to visit a friend who lives 90 km away. How fast must she go on the return trip if she wants her average sp

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Question 1003841: Super confused, need help by today-
Lisa travels at 30km/h on the way to visit a friend who lives 90 km away. How fast must she go on the return trip if she wants her average speed for the whole trip to be 40km/h? and 50km/h? and 60km/h?
Answer by josgarithmetic(39838) About Me  (Show Source):
You can put this solution on YOUR website!
That set of questions is for three different resulting values for what is the same variable.

Basic travel rates rule is RT=D to relate rate, time, and distance.

Setup a data table, fill in what is given, use variables if possible for what is not known, look for equations to form; solve the equations, equation, or system of equations as fits the question or objectives. 


             rate           time           distance

GOING         30            90%2F30=3         90

RETURN        r             90%2Fr            90

Total                       90%2Fr%2B3            180

The speed for the whole trip is 180%2F%2890%2Fr%2B3%29.



Let P be the average rate for the whole round trip.
highlight%28P=180%2F%2890%2Fr%2B3%29%29.
Expect P to be any given constant, positive value greater than 30; solve the formula for r.