1) 1/15 + ( 19/25 + 1/25 ) = 1/15 + 5/25 = 1/15 + 1/5 = = 1/15 + 3/15 = 4/15
2) ( 19/28 - 13/28 ) - 1/7 = 6/28 - 1/7 = 6/28 - 4/28 = 2/28 или 1/14
3) ( 7/16 + 5/18 ) + ( 1/16 + 1/18 ) = ( 7/16 + 1/16 ) + ( 5/18 + 1/18 ) = = 8/16 + 6/18 = 1/2 + 1/3 = 3/6 + 2/6 = 5/6
4) ( 10/21 + 4/21 ) + ( 19/42 - 5/42 ) = 14/21 +14/42 = 28/42 + 14/42 = 1
/ - черта дроби