Answer:
1.25 focal lengths
Explanation:
The lens equation states that:
[tex]\frac{1}{f}=\frac{1}{p}+\frac{1}{q}[/tex]
where
f is the focal length
p is the object distance
q is the image distance
In this problem, the image is 4 times as far from the lens as is the object: this means that
[tex]q=4p[/tex]
If we substitute this into the lens equation and we rearrange it, we get
[tex]\frac{1}{f}=\frac{1}{p}+\frac{1}{4p}=\frac{4+1}{4p}=\frac{5}{4p}\\p=\frac{5}{4}f=1.25 f[/tex]
so, the object distance measured in focal lengths is
1.25 focal lenghts
