Question 1194310
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Here is an informal alternative method for solving any 2-part "mixture" problem like this that is faster and easier than the standard algebraic solution shown by the other tutor -- if your mental arithmetic is good.<br>
You are mixing 20% sand and 100% sand to get a mixture that is 25% sand.<br>
Picture that as starting at 20% sand and moving towards 100% sand, stopping when you get to 25% sand.<br>
The "distance" from 20 to 100 is 80; the "distance" from 20 to 25 is 5; 5/80 = 1/16.<br>
In going from 20 towards 100 and stopping at 25, you have gone 1/16 of the total "distance".  That means the 100% sand you are adding is 1/16 of the mixture.<br>
That means the 420lb of mix that you started with is 15/16 of the mixture.<br>
Since 1/16 is 1/15 of 15/16, the amount of sand you need to add is 1/15 of 420lb, which is 28lb.<br>
ANSWER: 28 more pounds of sand<br>
The words of explanation make this method sound long and tedious, but it is not.  Without all the words, here are the calculations required to solve the problem:<br>
100-20 = 80; 25-20 = 5; 5/80 = 1/16
1-1/16 = 15/16
(1/16)/(15/16) = 1/15
420(1/15) = 28<br>