（1）2.23×2/5+2.77÷2.5
=2.23×2/5+2.77×2/5
=2/5×(2.23+2.77)
=2/5×5
=2
（2）7/8÷[1/3×(2/3-3/4)]
=7/8÷[1/3×(-1/12)]
=7/8÷(-1/36)
=7/8×(-36)
=-63/2