.3y+.5(200-y)=84?

0.3y + 0.5(200 - y)=84
=> 0.3y +100 - 0.5y=84
=> - 0.2y=84-100
=> - 0.2y= -16
=>0.2y=16
=>2y=160 [multiply by 10]
=>y=160/2
=>y=80<===answer

Should be written as :-
0.3 y + 0.5 (200 - y) = 84
3y + 5 (200 - y) = 840
3y + 1000 - 5y = 840
- 2y = - 160
y = 80

.3y +100-.5y=84
-.2y+100=84
16=.2y
y=80

0.3y + 0.5(200 - y) = 84
0.3y + 0.5*200 - 0.5*y = 84
0.3y + 100 - 0.5y = 84
10(0.3y + 100 - 0.5y) = 10(84)
3y + 1000 - 5y = 840
3y - 5y = 840 - 1000
-2y = -160
y = -160/-2
y = 160/2
y = 80