Bonjour,
S=-15/28÷(-25/42)
15/28x42/25
3/2x3/5
=9/10
63/44÷(-72/55)
-63/44x55/72
-7/4x5/8
=-35/32
12/45/16
12/1/45/16
12x16/1x45
4x16/15
=64/15
48/-32/21
48/-32/21/1
-48x1/32x21-48/32x21
-3/2x21
-1/2x7
=-1/14
-15/12+7/12÷5/18
-15/12+7/12x18/5
-15/12+7/2x3/5
-15/12+21/10
=17/20