Jawabanmu

2014-06-09T16:33:02+07:00
1/f = 1/so + 1/si
1/20 = 1/15 + 1/si
3/60 - 4/60 = 1/si
-1/60 = 1/si
-60 = si
2014-06-09T16:33:41+07:00