A double convex lens of focal length 15 cm is used as a magnifying glass in order to produce an erect image which is 3 times magnified. What is the distance between the object and the lens ?

Explanation

f = 15 cm
m = 3
Magnifying power `(m)=(-upsilon)/(u)=(f)/(f-u)`
`3=(15)/(15-u)`
`45-3u=15`
`3u=45-15`
`3u=30`
`u=(30)/(3)=10cm.`