(2.5+0.4)÷(1.25X0.8)=2.9÷1=2.9
(2又1/2+1÷2又1/2)÷(1.25×4/5)=(5/2+2/5)÷1=2.9
(5/2+2/5)÷(1.25X0.8)=2.9÷1=2.9