（1）3/4×4/9÷（4/5－3/10）
=1/3÷(8/10-3/10)
=1/3÷5/10
=1/3÷1/2
=2/3
（2）1/4+3×7/6
=1/4+7/2
=1/4+14/4
=15/4
（3）3/7×4/5+4/7×2/5
=4/5*(3/7+2/7)
=4/5*5/7
=4/7
（4）5/7÷3－2/21
=5/21-2/21
=3/21
=1/7