Using a certain concave mirror, the magnification is found to be 4 times as great when the object was 25 cm from the mirror as it was with the object at 40 cm from the mirror, the image, being real in each case.Find the focal length of the mirror.

Explanation

Given that m = 4
u = 25 cm
`m=(f)/(u-f)`
`4=(f)/(25-f)`
`100-4f=f`
`100 = 5f`
`f=(100)/(5)=20cm.`