Question 691001
We could start based on a total of {{{x}}} bulbs manufactured,
but we can calculate based on any total number of bulbs manufactured,
and the ratios would be the same anyway.
(I will choose 2000 because that gives me nice round numbers).
 
For every {{{2000}}} bulbs manufactured, the following is true:
Plant #1 makes {{{0.40*2000=800}}} bulbs (40% of {{{2000}}}), including
{{{(10/100)*800=80}}} defective bulbs.
Plant #2 makes {{{0.35*2000=700}}} bulbs (35% of {{{2000}}}), including
{{{(13/100)*700=101}}} defective bulbs.
Plant #3 makes {{{0.25*2000=500}}} bulbs (25% of {{{2000}}}), including
{{{(20/100)*500=100}}} defective bulbs.
 
Out of a total of {{{2000}}} bulbs, {{{80+101+100=281}}} bulbs are defective.
A customer who buys one bulb from that company has a
{{{281/2000}}} (or 14.05%) probability of getting a defective bulb.