Posted by Andrew on .
A convex lens forms a real image 6 times the size of the object. The distance between object and image is 33 cm. Find the focal length of the lens.

Physics 
drwls,
1/do + 1/di = 1/f
di/do = 6
di + do = 33
You have to solve three equations in three unknowns. Solve the last two in two unknowns first.
do + 6do = 33
do = 33/7 = 4.714 cm,
di = 28.286 cm
Now solve for f using the first equation.
f = 4.0406 cm