.
The fact that the sea water is 7.5% salt means that each liter contain 75 gram of salt.
Then 8000 liters of sea water contain 8000*75 = 600000 gram of salt.
In order to have concentration of 7% (or 70 gram of salt per liter) with this amount of salt, your volume of water must be
V = = 8571.4 liters.
So, you need to add 8571.4 - 8000 = 571.4 liters of fresh water.
Solved.
There are many ways to solve this problem, but this one is, probably, simplest.
It allows you to solve the problem without using equations (using the ratios only).
There is entire bunch of introductory 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 alloys
- Typical word problems on mixtures from the archive
in this site.
Read them and become an expert in solution 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 textbook in the section "Word problems" 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.