.
Let V be the volume of the pure alcohol to add.
Then the volume of the 30% mixture is (15+V) liters, and you have this equation
0.2*15 + V = 0.3*(15+V). (1)
It says that the pure alcohol amount in ingredients (left side) us the same as the alcohol amount in the final mixture (right side).
From this equation, you have
0.2*15 + V = 0.3*15 + 0.3V
V - 0.3V = 0.3*15 - 0.2*15
V = = 2.143 liters (approximately).
CHECK. I will check equation (1):
Left side is 0.2*15 + 2.143 = 5.143 liters of the pure alcohol;
Right side is 0.3*(15+2.143) = 5.143 liters of the pure alcohol.
! Precisely correct !
Solved.
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In this site, there is entire bunch of lessons covering various types of mixture problems
- Mixture problems
- More Mixture problems
- Solving typical word problems on mixtures for solutions
- Word problems on mixtures for antifreeze solutions
- Word problems on mixtures for dry substances like coffee beans, nuts, cashew and peanuts
- Word problems on mixtures for dry substances like candies, dried fruits
- Word problems on mixtures for dry substances like soil and sand
- Word problems on mixtures for alloys
- Typical word problems on mixtures from the archive
- Advanced mixture problems
- Advanced mixture problem for three alloys
- Unusual word problem on mixtures
- Check if you know the basics of mixtures from Science
- Special type mixture problems on DILUTION adding water
- Increasing concentration of an acid solution by adding pure acid
- Draining-replacing mixture problems
- How much water must be evaporated
- Advanced problems on draining and replacing
- OVERVIEW of lessons on word problems for mixtures
A convenient place to quickly observe these lessons from a "bird flight height" (a top view) is the last lesson in the list.
Read them and become an expert in solution the mixture word problems.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this online textbook under the topic "Mixture problems".
Save the link to this online textbook together with its description
Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson
to your archive and use it when it is needed.