Out of 1,100 discs tested, 13 are defective. Estimate the number of defective discs in a batch of 41,000.

This is a simple proportion

1100 : 13 = 41000 : x

For every 1100 discs, 13 are defective, meaning that there will be an X number for each 41000

13 * 41000 = 1100 * x

13 * 410 = 11 * x

5330 = 11 * x

x = 5330/11

x = 484.5

That means that for every 41000, approximately 485 discs will break down.