Refractive Index - Lens question!

1. The problem statement, all variables and given/known data



A double concave lens has equal radii of curvature of 15.1 cm. An object placed 14.2 cm from the lens forms a virtual image 5.0 cm from the lens. What is the index of refraction of the lens material?



A. 1.98

B. 1.9

C. 1.84

D. 2.06



2. Relevant equations



1/f = (n – 1 ) (1/R1 - 1/R2 )

1/f = 1/do + 1/di



3. The attempt at a solution



1/f = 1/14.2 + 1/5.0 so i get 1/f = 0.27



1/f = (n – 1 ) (1/R1 - 1/R2 )

0.27 = (n-1) (1/15.1 - (-1/15.1)



this results in an n equal to about 3.03 which is not close to any of the answers, though I'm sure i have the right equations, I'm just not sure what i'm doing wrong?

are these only applicable to convex lens?

1. The problem statement, all variables and given/known data







2. Relevant equations







3. The attempt at a solution