Question

At a light bulb factory, 3 out of every 1,000 bulbs produced are defective. If 5,000 bulbs are produced, how many would you expect to be defective?

Answer 1

3 out of 1000 are defective, so 3/1000 = 0.003 = 0.3% are defective

Apply 0.3% to 5000 to get...
0.3% of 5000 = (0.003)*5000 = 15

We expect about 15 bulbs to be defective (from the batch of 5000)

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Another way to look at this is to use the cross multiplication property

3/1000 = x/5000
3*5000 = 1000*x
15000 = 1000x
1000x = 15000
x = 15000/1000
x = 15
and we get the same answer