有一架天平只备有一个20克的砝码，要称出140克的物件，只许 称三次，应该怎样称？7N6T0B9y:z;P,f0w5h)H0N4V'`6W(A.o
算式：提示;[5r,X5g3~4n)S7@7N6T0B9y:z;P,f0w5h)H0N4V'`6W(A.o