So, the first brand has 9% vinegar, and say we use "x" amount of it, then, the amount of vinegar in "x" is 9% of x, or (9/100) * x, 0.09x.
the second brand has 14% of vinegar, and if we use say "y" amount of it, then the amount of vinegar in "y" is really just 14% of "y" or, (14/100) * y, 0.14y.

so, whatever "x" and "y" are, we know that
x + y = 230.
and we also know that their concentration amounts will also add up,
0.09x + 0.14y = 29.9.

how many milliliters will it be for the second brand? well, y = 230 - x.
I agree because he calculated it the right way and that is the same answer I got.
Hope I helped :)
13. 39
14. 2(x+4)
15. 24
16. 11(x+5)
17. 6(4x+3y)
18. 8(2x+10y)
19. 5(2x+3)
Hope this helped☺☺
I know it’s not right, I think it’s obtuse