设每支铅笔为x元,小虎买了21.6/x支铅笔
依题意列方程：
21.6/x+3=21.6/(x-0.1)
21.6(x-0.1)+3x(x-0.1)=21.6x
21.6x-2.16+3x^2-0.3x=21.6x
3x^2-0.3x-2.16=0
x={0.3±√[(-0.3)^2-4*3*(-2.16)]}/(2*3)
=[0.3±√(0.09+25.92)]/6
=(0.3±√26.01)/6
=(0.3±5.1)/6
x1=5.4/6=0.9
x2=-4.8/6=-0.8(不合题意,舍去）
21.6/0.9=24
答：小虎买了24支铅笔.