A:B=3:4
Let k be the common factor between A and B.
so, A =3k and B =4k.
and, B:C=5:7
Let x be the common factor between B and C.
so, B =5x and C =7x
∴4k=5x [Both are values of B]
x=4k5
A:B:C=3k:4k:7x
=3k:4k:7(4k5)
=15k:20k:28k5
=15:20:28 [on multiplying5kto each]
Hence A:B:C=15:20:28