Question 1117131
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You have received two responses to your question, both using an algebraic solution using an equation for the amount of sand in the mixture:<br>
"x kg of sand, plus 60% of 300 kg of mixture which is 60% sand, equals 75% of (x+300) kg of sand"<br>
This of course is a valid approach to the problem.  However, when one of the ingredients being mixed is 100% one thing and 0% the other, I find it easier to think in terms of the ingredient that is NOT being added.<br>
For this problem, you start with 300 kg of a mixture which is 40% cement and 60% sand, and you are adding only sand to end up with a mixture that is 75% sand and only 25% cement.<br>
(1) The cement in the original mixture is 40% of 300 kg, or 120 kg.
(2) When the additional sand is added to the mixture, that 120 kg is now only 25% of the mixture.
(3) So the total amount of mixture must now be 120/.25 = 480 kg.
(4) So the amount of sand that was added was 480-300 = 180 kg.