见解析
(1)0.75升=750立方厘米.
750×15÷25÷30,
=11250÷25÷30,
=15(厘米).
答:图①中D表示的数是15.
(2)30×15×15,
=450×15,
=6750(立方厘米),
=6.75升.
6.75÷15=0.45(升/秒),
0.75-0.45=0.3(升/秒).
答:从B洞中每秒钟流出0.3升水.
(3)25厘米=2.5分米,15厘米=1.5分米,24厘米=2.4分米,30厘米=3分米.
P点表示的数是:
(2.5+1.5)×3×2.4÷0.45,
=4×3×2.4÷0.45,
=64(秒),
Q点表示的数是:
(70-64)×0.45÷[(2.5+1.5)×3]+24,
=6×0.45÷[4×3]+24,
=6×0.45÷12+24,
=0.225+24,
=24.225(厘米).
答:P是64,Q是24.225.