(1)设甲种树苗购买x株,乙种树苗购买800-x株.
24x+30(800-x)=21000
24x+24000-30x =21000
-6x =-3000
x = 500
800-500=300(株)
答:甲种树苗购买500株,乙种树苗购买300株.
(2)设甲种树苗至多购买x株,乙种树苗购买800-x株
85% x+90%(800-x)≥800×88%
85 x+90(800-x)≥800×88
85 x+72000-90x ≥70400
-5x ≥-1600
x ≤320
答: 甲种树苗至多购买320株 .