where is the distance of the object in front of the lense and is the
distance of the image behind the lense and f is the forcal length of the lens
(for a thin converging lens)
In the first case we have:
and in the second:
where is the distance of the new image behind the lens.
Treat and as the variables and we have a pair of simultaneous
linear equations to solve, which will allow us to find .