DEFECTIVE LIGHT BULBS.....

[smclean23]

One-fifth of the light switches produced by a certain factory are defective. Four-fifths of the defective switches are rejected and 1/20 of the nondefective switches are rejected by mistake. If all the switches not rejected are sold, what percent of the switches sold by the factory are defective?
(A) 4%
(B) 5%
(C) 6.25%
(D) 11%
(E) 16%

[iamabhisheksingh]

20% of the switches are defective. So only 80% are sold. Now 20% of the original switches are rejected.

So 20/80 * 100 is 5%.

[dbart06]

How do you get 20/80 * 100 = 5%

20/80 is 1/4 * 100 = 25%

[dbart06]

smclean23,

I found the answer by making a chart: i used 100 as total

def non def sold
rej 16 4
non rej 4 76 80

total sold 80 & the amt def sold is 4 which is 4/80=5%

[hakyology]

use 100 for the total number. and
1/5 times 100 = 20 (defective) so, 80 is nondefective.
4/5 times 20(defective ones) = 16 (rejected)
therefore, only 4 are defectve out of 20 defective ones.

1/20 times 80(nondefective ones) = 4 (rejected)
so the rest is 76(nondefective ones) left.

so, 4/80 times 100 is 5 percent.

i hope it helps.