A converging lens is used to magnify a virtual image to four times it original size. Given then focal length of the converging lens is 20.0 cm, what is the distance of the object from the lens?

m=v/u

4=v/u   =>  v=4u

But image is virtual, so v must be negative (ie. v = -4u)

1/f = 1/u +1/v

1/f = 1/u + 1/(-4u)

1/20 = (4-1)/4u

u = 15 cm