#### Dx

I have 2 problems that I need some help with, plz.

1) How far from a 50mm focal length lens, such as used in many 35mm cameras, must an object be positioned if it is so to form a real image magnified by a factor of 3?

how do i solve for this? I have tried to solve using lens equation but not the answer I would expect, my answer is 52mm. Is this correct if not how did you go about solving for this?

2) A biconvex lens is formed by using a piece of plastic(n=170). the radius of the front surface is 20cm and the radius of the back surface is 30cm. what is the focal length of the lens?

I know that r/2 = f but should i add both radius's and use that /2? I need some help?

Thanks!

Dx