Suzie (of mass 70 kg) is roller-blading down

Question

the sidewalk going 20 miles per hour. She
notices a group of workers down the walkway
who have unexpectedly blocked her path, and
she makes a quick stop in 2.2 seconds.
What is Suzie’s average acceleration?
1. -34.2657
2. -3.43803
3. -4.46944
4. -1.98642
5. -134.083
6. -15.8914
7. -4.06313
8. -5.84466
9. -7.44907
10. -62.5722
Answer in units of m/s
2

Answer 1

(-20mi/hr)/2.2s * 1609m/mi * 1hr/3600s
= (-20*1609)/(2.2*3600) m/s^2

Answer 2

Vo = 20mi/h * 1600m/mi * 1h/3600s = 8.9 m/s.
V = Vo + a*T = 0,
8.9 + a*2.2 = 0,
a = -4.04 m/s^2.