(7/4十14/5)÷21/20
=(7/4十14/5)×20/21
=7/4×20/21+14/5×20/21
=5/3十8/3
=13/3
用到乘法分配律
(7/4+14/5)÷21/20
=(7/4+14/5)*20/21
=7/4*20/21+14/5*20/21
=5/3+8/3
=13/3=4又1/3
(7/4+14/5)÷21/20
=(35+56)/20×20/21
=13/3
(7/4+14/5)÷21/20
=7/4×20/21+14/5×20/21
=5/3+8/3
=13/3