Focal length of plano-convex lens be f1
‌

1

f1

‌=(1.5−1)(‌

1

15

−‌

1

∞

)
Or,‌

1

f1

‌=‌

1

30

∴ f1‌=30‌cm
Focal length of plano-concave lens be f2
‌

1

f2

‌=(1.5−1)(‌

1

∞

−‌

1

15

)
∴‌‌f2‌=−30‌cm
since, parallel rays are incident on the plano convex lens it will form an image at a distance 30‌cm from the lens (since, focal length is 30‌cm ).
This image will act as an object for the plano concave lens.
Object distance =30−20=10‌cm
Applying lens formula,
‌

1

v2

−‌

1

u2

‌=‌

1

f2

‌ Or, ‌‌‌‌

1

v2

−‌

1

10

‌=−‌

1

30

So, v2=15‌cm