Jarak bayangan yg terbentuk

1/f= 1/s+1/s'
F= jarak fokus
s= jarak benda
s'= Jarak bayangan
1/20 = 1/30 + 1/s'
1/s' = 1/20 - 1/30
       = 3/60 - 2/60
       = 1/60
s' = 60/1
   = 60 cm (C)
Maaf klau ada kesalahan