When an object is placed at a distance of 25 cm from a concave mirror, the magnification is ${\text{m}}_{\text{1}}$ . The object is moved 15 cm farther away with respect to the earlier position, and the magnification becomes ${\text{m}}_{\text{2}}$ . If ${\text{m}}_{\text{1}}{\text{/m}}_{\text{2}}=4$ the focal length of the mirror is (Assume that image is real and ${\text{m}}_{\text{1}}{\text{,m}}_{\text{2}}$ are numerical values)