[1/15 + 1/21 + 1/20] : 1 2/21= 1/15 +1/21 + 1/20]:23/21=
aflam numitorul comun
15=3x5
21=3x7
20=2^2x 5 (2 la a 2 ori 5)
c.m.m.mc [15.20.21]=3x5x4x7=420
420:15=28
420:20=21
420:21=20
vom amplifica fractiile
[1/15 + 1/21 + 1/20] : 1 2/21= 1/15 +1/21 + 1/20]:12/21=[28/420 +20/420 +21/420] : 240/420=28+20+21 / 420 : 240/420=69 / 420 : 240/420=69 / 420 x420 / 240=69 x 420 / 420x240 = 28980 / 100800 simplificam cu 10
=2898 / 10080 simplific cu 9
=322 / 1120 simplific cu 2
=161 / 560