Calculating with the lens formula:

Using the real is positive convention:
Where a value is real it is regarded as positive. Where it is virtual it is taken as negative.
Virtual images, objects and  focal lengths all exist.  
Eg the focal length of a concave lens is virtual.
And the virtual image produced by a lens can be the “object” of a second lens.
	Focal length of lens
cm	Object  distance from lens axis
cm	Image distance from lens axis
cm	
1	25.0	30.0		
2		15.0	15	
3	20.0		-10	
4	12.0		16	
5		7.0	-3	

In each case state whether the image is 
a)	magnified or diminished  b) upright or inverted c) real or virtual:
1 a)_______________  b) _________________   c)__________________
2 a)_______________  b) _________________   c)__________________
3 a)_______________  b) _________________   c)__________________ 
4 a)_______________  b) _________________   c)__________________
5  a)_______________  b) _________________   c)__________________
Calculate the magnification of image
1_____________________   2_____________________           3___________________

4_____________________  5 ______________________