Find the displacement in a word problem

[Natko]

Homework Statement

A car is slowing down at a rate of 20 km/h/s. How far does it travel if its original velocity is 50 km/h and its final velocity is 5.0 m/s?

Homework Equations

v_f² = v₀² + 2aΔx

The Attempt at a Solution

v_f² = v₀² + 2aΔx
Δx = (v_f² - v₀²) / 2a
= (18²-50²) / 2(-20)
= -2176 / -40
= 54.4 m

That's wrong, though. The answer should be 15 m.

 

[Natko]

Whoops, forgot to convert km/h/s to km/h/h. I got the answer now :)

Homework Statement

A car is slowing down at a rate of 20 km/h/s. How far does it travel if its original velocity is 50 km/h and its final velocity is 5.0 m/s?

Homework Equations

v_f² = v₀² + 2aΔx

The Attempt at a Solution

v_f² = v₀² + 2aΔx
Δx = (v_f² - v₀²) / 2a
= (18²-50²) / 2(-20)
= -2176 / -40
= 54.4 m

That's wrong, though. The answer should be 15 m.