Jawabanmu

2014-06-06T11:58:10+07:00
1/20 = 1/30 + 1/s'
-1/s' = 1/30 - 1/20
-1/s' = 2/60 - 3/60
1/s' = 1/60
s' = 60 cm
semoga membantu
maaf ada kesalahan
fokusnya negatif jadi s' = 12