Question

Where should an object be placed before a concave mirror a focal length 20 CM so that the real image is formed a distance of 60 CM from it

Answer 1

for a real image, image distance must be of same sign as that of object's distance.

Considering the object to be on positive side, and by using mirrors formula
$\frac{1}{u} = \frac{1}{f} - \frac{1}{v}$
So, that means
$\frac{1}{u} = \frac{1}{20} - \frac{1}{60}$
$\frac{1}{u} = \frac{3 - 1}{60}$
$u = 30cm$

Answer 2

HEY!!

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✔Image Distance (v) = -60 cm and focal length (f) = -20 cm

✔Object Distance (u) = ?

✔By using the mirror formula,

✔1/v + 1/u = 1/f

✔1/-60 + 1/u = 1/-20

✔1/u = -1/20 + 1/60

✔By taking the LCM

✔1/u = [(-1*3) + 1]/60

✔1/u = -2/60

✔1/u = *1/30

✔u = -30 cm

▶▶So, the object must be placed between Centre of Curvature and Focus at the distance of 30 cm from the mirror.