when an object is placed 10cm away from a lens; it is found that the formed is 6cm behind the object on the same side of the lens.

Determine the focal lenght of the lens.

solution
1/v + 1/u = 1/f

u=10cm, v=6cm, f=?

6+10/60 = 1/f

70/60 = 1/f

by cross multiply we have
70 x f = 60 x 1

70 x f = 60

f = 60/70 = 6/7 ans.