In an electric circuit, two resistors with resistances x and y are connected in parallel. If R is the combined resistance of these two resistors, then the reciprocal of R is equal to the sum of the reciprocals of x and y. What is R if x is 4 Ohms and y is 5 Ohms?	Answers: A) 7/9 B) 20/9 C) 7/20 D) 9/20 E) 11/20
1/R = 1/x + 1/y   1/R = 1/4+1/5 = 9/20   R = 20/9   The answer is 20/9.
<result>B</result>
In an electric circuit, two resistors with resistances x and y are connected in parallel. If R is the combined resistance of these two resistors, then the reciprocal of R is equal to the sum of the reciprocals of x and y. What is R if x is 4 Ohms and y is 5 Ohms?