Question

Each cone of the hourglass has a height of 15 millimeters. The total height of the sand within the top portion of the hourglass is 45 millimeters. The radius of both cylinder and cone is 6 millimeters. Sand drips from the top of the hourglass to the bottom at a rate of 10π cubic millimeters per second. How many seconds will it take until all of the sand has dripped to the bottom of the hourglass?

Answer 1

We have to calculate the height of a cylinder : h2 = ( 45 - 2 · 15 )/ 2 = 15 / 2 = 7.5 mm
Volume of the bottom of the hourglass:V = 1/3 r² π h1 + r² π h2 = = 1/3 · 6² · 3.14 · 15 + 6² · 3.14 · 7.5 = = 1/3 · 36 · 3.14 · 15 + 36 · 3.14 · 7.5 = = 565.2 + 847.8 = 1,413 mm³The rate is 10 π mm³/ sec = 31.4 mm³/secTime = 1,413 : 31.4 = 45 sec.Answer: It will take 45 seconds.