
Elementary Math Fractions, Percentages, Word Problems, Equations, Inequations, Factorization, Expansion 
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April 13th, 2018, 02:34 AM  #1 
Senior Member Joined: Aug 2014 From: India Posts: 294 Thanks: 1  Why we consider M in first problem only?
In a milk solution of 10 lit, 2 lit of water is added thereby the concentration of milk is reduced by 15%. What is the quantity of milk in the solution? (Ans is 9) Here we consider M be the amount of milk and we proceed like this: $\dfrac{M}{10}\dfrac{M}{12}=0.15$ But here in this problem: A Milk solution of 60 litres contains 80% milk. How many litres of another milk solution containing 90% milk should be added to make an 84% milk solution? We skip this M be the amount of milk and proceeded in totally different way: $$60\times\frac{80}{100}+n\times\frac{90}{100}=(60 +n)\times\frac{84}{100}$$ Why we consider M be the amount of milk in first problem only? Last edited by Ganesh Ujwal; April 13th, 2018 at 02:37 AM. 
April 13th, 2018, 02:57 AM  #2 
Math Team Joined: Jan 2015 From: Alabama Posts: 3,240 Thanks: 885 
In the first problem, you are specifically asked to find "the quantity of milk". It is natural to assign a name to the thing you are trying to find. In the second, you are asked to find "How many litres of another milk solution", NOT the amount of milk. Here you have assigned "n" to that amount.

April 13th, 2018, 07:34 AM  #3 
Senior Member Joined: Aug 2014 From: India Posts: 294 Thanks: 1  