Question 1019523
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1) How many coins, each radius 0.75 cm and thickness 0.2 cm, have to be melted to make a right circular cylinder 
of height 8 cm and base diameter 6 cm?

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The diameter of each coin is 1.5 cm, and you need {{{(6/1.5)^2}}} = {{{4^2}}} = 16 coins to get an equivalent base area.

Then you need {{{8/0.2}}} = 40 layers of coins of thickness 0.2 cm to get the height of 8 cm.

Now multiply 16*40 = 640, and you will get the required number of coins.
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2) Water is being pumped into a water tank of dimensions 10 m X 7.5 m X 4 m at the rate of 400 L per minute. 
What is the time taken to fill the tank complete?

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Volume of the tank  10*7.5*4 = 300 {{{m^3}}}  divide by the rate  0.4 {{{m^3/minute}}}:   {{{300/0.4}}} = 750 minutes = {{{12}}}{{{1/2}}}  hours.
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