Cookie Question: Ratio of Diameter to Thickness and Uncertainty

[Hi.O303]

I saw similar types of problems like this using Google search, but none of them answered this question specifically.

Homework Statement

As you eat your way through a bag of chocolate chip cookies, you observe that each cookie is a circular disk with a diameter of 8.5 +/- 0.02 cm and a thickness of 0.050 +/- 0.005 cm.

Find the ratio of the diameter to the thickness and the uncertainty in this ratio.

Homework Equations

The Attempt at a Solution

So what I did was just put 8.50 +/- 0.02 cm divided by 0.050 cm. My answer was :
170 +/- 4cm

Their answer was : 170 +/- 20 cm.

So I see where they got the 170 from, but I'm not sure why they 20cm is there.

 

[Anden]

In such questions, try to get the biggest ratio possible and the smallest one.

 

[Hi.O303]

Okay, I did that and I got the two answers : 154.9 and 188.4, but the difference between them isn't 20.

 

[Redbelly98]

They seem to be rounding the error to one significant figure.

Have you been told about adding the percent error when doing multiplication or division?

EDIT:

Okay, I did that and I got the two answers : 154.9 and 188.4, but the difference between them isn't 20.

Just FYI, those numbers are a little off. For example,
8.48/0.055 = ____? (slightly less than the 154.9 you gave)