設v為泳池容量,x為幼水喉流量,y為粗水喉流量。t為只用粗水管的時間
V=(x+y)4=4x+4y -------(1)
y(t)=x(t+6)=V------(2)
(t)(y-x)=6x-----(3)
put (2) into (1), we have
y(t)=4(x+y)-----(4)
from (3), we have x=yt/(t+6)----(5)
put (5) into (4)
y(t)= 4yt/(t+6) +4y
y(t)(t+6) = 4yt +4y(t+6)
t^2+6t=4t+4t+24
t^2-2t-24=0
(t-6)(t+4)=0
t=6 or -4(reject)
因此 t=6小時