Answer:
45 cm
Explanation:
The formula to apply in this question is that of object distance , image distance and focal length.
This is written as;
[tex]\frac{1}{f} =\frac{1}{o} +\frac{1}{v}[/tex]
where;
f= focal length = 30cm
o= object distance = 90 cm
v= image distance = ?
Applying real values as;
1/30 = 1/90 + 1/v
1/30 - 1/90 = 1/v
1/45 = 1/v
v= 45 cm
