Question

what is it focal length of double conacve lens kept in air with two spherical surfaces of radii r1=30xm and r2=60 cm take refractive index of lens as n=1.5

Answer 1

I think so the answer might be like this way i am not sure
we know that ------    F = R by  2
where R is radius  and F = focal length 
 therefore      first concave lens it must be 
F = 30 by 2
ie = 15 cm 
and the second 
F =60 by 2
ie = 30 cm