设甲书架有x册,则乙有3000-x册
2x/5-420=(3000-x)/4
x=1800
则甲有260册,乙有3000-1800=1200册
设甲为X,乙为Y
X+Y=3000
2/5X-1/4Y=420
解:X=1800
Y=1200
答:甲书架有书1800册,乙书架有书1200册.
设甲书架有x册,则乙有3000-x册
2/5x=(3000-x)/4
x=1800,y=1200
甲:3000-1200=1800(本)乙:(3000x2/5-420)÷(2/5+1/4)=1200(本)