(700*a+200)-600=1000
700a+200-600=1000
700a-400=1000
700a=1000+400
700a=1400
a=2
=============
(7200-a*200)+2000=3400
(7200-200a)+2000=3400
7200-200a+2000=3400
9200-200a=3400
-200a=3400-9200
-200a=-5800
a=29
==============
(y:4-35)*120=3000
(y/4-35)*120=3000
30y-4200=3000
30y=3000+4200
30y=7200
y=240
==============
(50*y+160)-60=500
50y+160-60=500
50y+100=500
50y=500-100
50y=400
y=8