5/(7/9)/(9/11)/(11/21)
= 5÷7/9÷9/11÷11/21
= 5×9/7×11/9×21/11
=5×（9/7×11/9）×21/11
=5×11/7×21/11
=5×（11/7×21/11）
=5×3
=15