Ответ:
1) (1/2+1/3)×(1/5-1/4)= 5/6×(-1/20) , -1/6×1/4= -1/24
2) (2/7-3/4)×(2/13-1/2)= -13/28×(-9/26), 1/28×9/2=9/56
3) (0,3-1/20):(3/4-1,25)=(3/10-1/20)÷(3/4-5/4) , 1/4÷(-1/2), -1/4×2= -1/2
4) (2,7-1/5):(1/2+4,5)= 3/35÷(1/2+9/2) , 3/35÷5 , 3/35×1/5=3/175