Question

an object 15 cm from a lens produces a real image 30 cm from the lens what is the focal length of the lens​

Answer 1

f=-30cm is the answer of question

Answer 2

real image can only be produced by convex lens

so

v= +30 and u= -15

formula of lens---> 1/f=1/v-1/u

now putting values

1/f= 1/30-1/-15

1/f= 1/30+1/15

1/f = 3/30

f=10 cm

hope it helps !!!