(2/7÷4/9+1/2)x14/41
=(9/14+1/2)x14/41
=9/14x14/41+1/2x14/41
=9/41+7/41
=16/41
第二题
3÷(1-80%)=3÷20%=15
15x80%=12
答 甲数是12