Given that

$$ Total\quad volume=25 \ liters $$

$$ Percentage\quad of\quad solvent=40\% $$

$$ Percentage\quad of\quad water=60\% $$

So,

$$ Volume\quad of\quad solvent=\frac{40}{100}\times25 $$

$$ =10\quad liters $$

$$ Volume\quad of\quad water=\frac{60}{100}\times25 $$

$$ =15\quad liters $$

$$ Liters \quad to\quad add=15-10=5 \quad liters $$