Vlad77 wrote:

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%

Rejected defective switches = 1/5 x 4/5 = 4/25

Accepted defective swicthes = 1/5 x 1/5 = 1/25

Rejected good switches = 4/5 x 1/20 = 1/25

Accepted good switched = 4/5 x 19/20 = 19/25

Since all accepted switches are sold so total switches sold = 1/25 + 19/25 = 4/5

required answer = 1/25 / 4/5 = 1/20 = 5%