image in a concave mirror

kateman

Where must an object be placed if the image formed by a concave mirror is to be 1/4 as far from the mirror as the object and is the image real or virtual?

Di= 1/4 Do

1/Do - 1/Di = 1/f
1/Do -1/(1/4)Do = 1/f
1/Do-4/Do =1/f
-3/Do = 1/f
Do= -3/1/f=-3/f

this is wrong as the answer in the back of the book is: 2|f|
this answer absolutely confused me and i cant think what i did wrong, normally reflection/refraction is really straight forward to me.
Any advice? thanks :)