Question

from 40 bulbs tested by a quality controller, 3 are faulty calculate the expected number of faulty bulbs in a box of 500

Answer 1

Out of 40 bulbs, 3 were faulty.

3/40 were faulty

out of 500 bulbs, 'x' were faulty.

x/500 were faulty

$\frac{3}{40} = \frac{x}{500}$
cross multiply
40x=1500
Divide both sides by 40
x=37.5
Since you can't have .5 of a bulb, lets round up to 38
Answer=38 bulbs