某数比它的4倍大3
x-4x=3
-3x=3
x=-1
某数与6的和的3倍等于21
3(x+6)=21
x+6=7
x=1
某数与4的和的3倍等于7
3(x+4)=7
x+4=7/3
x=-3/5
某数的7倍比该数大5
7x-x=5
6x=5
x=6/5
某数的3/1比6小1
x*3/1=6-1
x*3/1=5
x=15
某数的80%与19的和的3/2等于10
(x*80%+19)*3/2=10
x*80%+19=15
x*80%=-4
x=-5