Question

Mr. Donley drove 504 km in each direction on a vacation

trip. The average rate going was 14 km/h faster than the
rate returning. The total time for the entire trip was 21
hours. Find the average rate for each direction.

Answer 1

Data that we know:

He drove 504km in each direction.

Going, the average speed was 14km/h more than returning.

The total trip lasted 21 hours.

Ok, to solve this first:

Let's define the variables:

Sg = Average speed when going.

Sr = Average speed when returning.

Then we can write one of our relationships as:

Sg = Sr + 14km/h.

Now, you can recall the relation:

Time = Distance/speed.

Then we can write the equation that represents the total time of the travel as:

504km/Sg + 504km/Sr = 21h

Now we can replace the Sg by Sr + 14km/h, and get:

504km/(Sr + 14km/h) + 504km/Sr = 21h.

Now we must multiplacate by each denominator, in order to remove them:

504km*Sr + 504km*(Sr + 14km/h) = 21h*Sr*(Sr + 14km/h)

Sr*1008km + 7056km^2/h = 21h*Sr^2 + 294km*Sr

Now we can write a cuadratic equation, where i will ignore the units so it is easier to read, as:

21*Sr^2 -714*Sr - 7056 = 0

The solutions are given by the Bhaskara formula:

`Sr = (714 +- √(714^2 - 4*21*(-7056)) )/(2*21) = (714 +- 1050)/(42)`

We have two solutions, one negative and one positive, and because Sr represents an average speed, it must be a positive number, then we choose the positive solution:

Sr = (714 + 1050)/42 km/h = 42km/h

Now we have the average speed for the returning, with this we can find Sg.

Sg = Sr + 14km/h = 42km/h + 14km/h = 56km/h.