Question 1194310
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<pre>
420 lbs of the 20% sand mixture contains 0.20*420 = 84 lbs of sand.


Hence,  420-84 = 336 lbs is another material (not sand): break stone and cement.


We want 336 lbs be 75% of the new mixture;  hence, the total new mixture is  {{{336/0.75}}} = 448 lbs.


It means that  448-420 = 28 lbs of sand should be added.    <U>ANSWER</U>
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Solved mentally, &nbsp;without using equations &nbsp;(only calculator :)