A factory makes lightbulb's out of 400 light bulbs 18 were found to have defective filaments

If you are asking for percentage, 18/400 bulbs were defective. In other words, 4.5% of bulbs were defective. This also means that 95.5% of bulbs were not defective.

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step-by-step explanation:

we need to solve for 'x' the following equation:  

we have that:

⇒

⇒ ⇒

factorizing, we get that:

  ⇒

then, the solutions are: or

nothing extraneous has been found!

answer: x=60