Two factory plants are making tv panels. yesterday, plant a produced 8000 panels. one percent of the panels from plant a and 4% of the panels from plant b were defective. how many panels did plant b produce, if the overall percentage of defective panels from the two plants was 3% ?

8000 from plant A with 1% defective
x        from plant B  with 4% defective
8000 + x = y           with 3% defective

da/a = 0.01 -> 80 = da 
db/b = 0.04 -> db = 0.04b
(da+db)/(a+b) = 0.03

(80+b*0.04)/(8000+b) = 0.03
80+b*0.04 = 0.03*(8000+b)
80+b*0.04 = 240 + 0.03b
80 - 240 = 0.03b - b*0.04
-160 = -0.01b
b = 160/0.01
b = 16000

the panels from the plant b were 16000 

No because if you do the higher number minus the lower number you will get a positive. if you do it the other way around you will get a negative number