Question

12. A cyclist travels 40 km at a speed of x km/h. Find the time

taken in terms of x. Find the time taken when his speed is
reduced by 2 km/h. If the difference between the times is
1 hour, find the original speed x.

Answer 1

the answer is 10 km/hr.

Answer 2

Explanation:

distance = rate times time, so

distance

time = --------------

rate

Here the distance is 40 km and the rate is x. Then

40 km

time = -------------- = (40/x) hr

x km/hr

If the speed is reduced by 2 km/hr, we get:

40 km 40 km 40

time = ------------------------------- or time = ----------------------- = --------- hr

(x km/hr - 2 km/hr) (x - 2) (km/hr) x - 2

The difference between the times is:

40 40

-------- - --------- = 1 hr Solve this for the original speed, x

x - 2 x

40x - 40x + 80 80

----------------------- = 1 hr or ------------ = 1 or 80 = x^2 - 2x

x(x - 2) x(x - 2)

Rewriting this in standard quadratic form: x^2 - 2x - 80 = 0

Here a = 1, b = -2 and c = -80, and so the discriminant b^2 - 4ac is:

(-2)^2 - 4(1)(-80) = 324

The solutions are:

-(-2) ± √324 2 ± 18

x = -------------------- = ------------ = 1 ± 9, or 10 (discard the other root)

2 2

The original speed was 10 km/hr