a] 2,8+4,7+7,2+5,3
b] 12,34 +23,87+7,66+32,13
c] 45,09 +56,73 +54,91 +43,27
d] 12,23 + 24,47+ 31,18+ 63,3+ 68,82
a] 12,3 + 34,56 + 8,7
= [ 12,3 + 8,7 ] + 34,56
= 21 + 34,56
= 55,56
b] 12,56 + 34,46 + 87,44 + 5,54
= [ 12,56 + 87,44 ] + [ 34,46 + 5,54 ]
= 100 + 40
=140