Answer:
f = 6.66 cm
Explanation:
For this exercise we will use the constructor equation
1 / f = 1 / p + 1 / q
where f is the focal length, p is the distance to the object and q is the distance to the image
the expression for magnification is
m = h '/ h = - q / p
with this we have a system of two equations with two unknowns, in the problem they give us the distance to the image q = 10 cm and a magnification of m = -0.5
-0.5 = - q / p
p = - q / 0.5
p = - 10 / 0.5
p = 20 cm
now we can with the other equation look for the focal length
1 / f = 1/20 + 1/10
1 / f = 0.15
f = 6.66 cm
