Find the focal length of a concave mirror if it produces two times large image of an object placed at 20cm from it

 solution:- Given that mirror is concave so 

Object distance(u)= Negative(According to sign convention)

u=-20 cm 

magnification=-2

m=-v/u

-2 = -v/-20

v=-40(means image real and inverted)

now from mirror formula,

1/v + 1/u = 1/f 

1/-40 + 1/-20 =1/f 

1/f = {-1+(-2)}/40

1/f = -3/40

f= -40/3 

f = -13.33 cm ans