（1）[4/5]+[5/9]+5+[4/9]
=[4/5]+（[5/9]+[4/9]）+5
=[4/5]+1+5
=[4/5]+6
=6[4/5]
（2）[13/5]-[2/7]-[5/7]
=[13/5]-（[2/7]+[5/7]）
=[13/5]-1
=[8/5]
=1[3/5]
（3）[1/8]+[8/15]+[7/8]-[3/15]
=（[1/8]+[7/8]）+（[8/15]-[3/15] ）
=1+[1/3]
=1[1/3]
（4）4.5-（4.25-[1/2]）
=4.5-（4.25-0.5）
=4.5-4.25+0.5
=4.5+0.5-4.25
=5-4.25
=0.75