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a) -3/5={-2/3+[3/4-2/5)-(4/5-7/4)]-7/5}
b)-3+{[-(3/10-0,7)+(-7/5-6)]-(4/5-2)]
c) [13/20-(12/15-3/4)]-[7/20+(9/4+15/21)]
#Relede