Bad Question - Average Acceleration

[Qube]

Homework Statement

A car starts from rest. With constant acceleration, the car increases its speed from 10m/s to 50m/s over a distance of 60m. How long does this take?

Purported answer: 4.0s

Homework Equations

Constant acceleration, so the following equations apply:

Vf^2 = Vi^2 + 2ad
Vf = Vi + at

The Attempt at a Solution

Vf^2 = Vi^2 + 2ad

2500 = 100 + 2a(60)
2400 = 120a
240/12 = a
a = 20 m/s^2.

Vf = Vi + at

50 = 10 + 20(t)
40/20 = 2s.

Why does the key say 4s?

I'm guessing it's because the key is wrong!

 

[Dick]

Homework Statement

A car starts from rest. With constant acceleration, the car increases its speed from 10m/s to 50m/s over a distance of 60m. How long does this take?

Purported answer: 4.0s

Homework Equations

Constant acceleration, so the following equations apply:

Vf^2 = Vi^2 + 2ad
Vf = Vi + at

The Attempt at a Solution

Vf^2 = Vi^2 + 2ad

2500 = 100 + 2a(60)
2400 = 120a
240/12 = a
a = 20 m/s^2.

Vf = Vi + at

50 = 10 + 20(t)
40/20 = 2s.

Why does the key say 4s?

I'm guessing it's because the key is wrong!

Yes, I think the key is wrong.

 

[D H]

I concur with Dick. The key is wrong. Why? They might have changed the question from some previous edition and not changed the key. Or they might have solved a problem other than the one printed. Or they might have just made a mistake. It happens.