1、设甲有x株,乙500-x株
50x+80(500-x)=28000
50x+40000-80x=28000
30x=12000
x=400
500-400=100
2、
50x+80(500-x)≤34000
30x≥6000
x≥200
3、x×90%+(500-x)×95%≥500×92%
0.9x+475-0.95x≥460
0.05x≤15
x≤300
W=50x+80(500-x)
=50x+40000-80x
=40000-30x
x值越大,费用越低
所以当x=300时,费用最低