What is the value of the focal length fn2

Question

of a convergent lens with index of refraction ( n2 =1.5) if it was
fn1 =5cm for an index of refraction n1 = 1.4 .

Answer 1

f is inversely proportional to 1/(N-1).

fn2/fn1 = .4/.5 = 0.8

fn2 = 4 cm