Mechanical Engineer, four cavity mold, how many parts to test?


New Member
My friend is a mechanical engineer at a company in CT. I was pretty good at stats back in my day so he asked me to solve this for him. This is a REAL question he is facing for his company/customers.

There are 200(+- a few) parts in a bin. The parts were made from a four cavity
mold. There is no way to tell which cavity each part came from. Before
the company can ship any parts to the customer they need to test at
least 3 parts that were produced from each cavity. How many parts does
the company need to test so that with 90% confidence they have tested at least 3 parts from each of the four cavities?

Please explain/show your work so I can understand it, and so he can use the formula himself next time. Also, I'm a high school math teacher, and this might make a fun class activity.

Thank you!