Thank you for reporting, we will resolve it shortly
Q.
A convex lens and a concave lens of focal lengths $15\, cm$ and $20\, cm$ respectively are placed in contact with each other. When an object is placed at $60\, cm$ from the combination, image is formed at
Solution:
Combined focal length
$\frac{1}{f}=\frac{1}{15}+\frac{1}{(-20)}=\frac{4-3}{60}$
$f=60\, cm$ ( a convex lens)
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{v}+\frac{1}{60}=\frac{1}{60}$
$v=\infty$ (image at infinity), because object is at focus of convex lens.
