3sur5 + 1sur15 =  ? 25sur8+2.5sur4 = ? 21sur9-1sur3 25sur3-64sur12 = ? Je dois simplifier et calculer

10 sur 15 
30 sur 8
18 sur 9
36 sur 12

3/5 + 1/15 = 3x3/5x3 + 1/15 = 15 = 9/15 + 1/15 = 10/15
25/8 + 2.5/4 = 25/8 + 2.5x2/2x4 = 25/8 + 5/8 = 30/8 
21/9 - 1/3 = 21/9 - 1x3/3x3 = 21/9 - 3/9 = 24/9
25/3 -64/12 = 25x4/3x4 - 64/12 = 100/12 - 64/12 = 36/12 = 3