Question 839697
Very typical exercise for a two part mixture.


u = volume of 15% salt
v = volume of 25% salt


{{{(15u+25v)/10=21}}} and {{{u+v=10}}}


This is a system of two linear equations in two unknown variables.  Solve for u and v.


See lesson:
http://www.algebra.com/algebra/homework/word/mixtures/Mixtures%3A-All-in-Symbols.lesson