Дано
F=12см
k=2
d-?
________________
1/F =1/f+1/d
k=f/d
f=kd
1/F=1/kd+1/d
1/F=1+k/kd
1/F=3/2d
2d=3F
d=3F/2
d=3×0,12/2=0,18м =18 см