For lens L1
u = −15cm
f = +20 cm
using lens formula
\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)
\(\frac{1}{v}=\frac{1}{u}+\frac{1}{f}\)
\(\frac{1}{v}=-\frac{1}{15}+\frac{1}{20}=-\frac{1}{60}\)
v = −60 cm
This image will act as object for lens L2 lens
u = −20 − 60 = −80cm
v = + 80 cm
\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)
\(\frac{1}{80}+\frac{1}{80}=\frac{1}{f}\)
\(\frac{1}{f}=\frac{1}{40}orf=40cm\)